Fix multithreading usage of template rendering

[SergiusTheBest]

The call stack of the issue:

java.lang.NullPointerException: Cannot invoke "String.startsWith(String)" because "this.doctypeLine" is null
        at de.neuland.pug4j.template.PugTemplate.setDoctype(PugTemplate.java:73) ~[pug4j-2.2.0.jar:na]
        at de.neuland.pug4j.parser.node.DoctypeNode.execute(DoctypeNode.java:12) ~[pug4j-2.2.0.jar:na]
        at de.neuland.pug4j.parser.node.BlockNode.execute(BlockNode.java:32) ~[pug4j-2.2.0.jar:na]
        at de.neuland.pug4j.compiler.Compiler.compile(Compiler.java:31) ~[pug4j-2.2.0.jar:na]
        at de.neuland.pug4j.template.PugTemplate.process(PugTemplate.java:28) ~[pug4j-2.2.0.jar:na]
        at de.neuland.pug4j.PugConfiguration.renderTemplate(PugConfiguration.java:96) ~[pug4j-2.2.0.jar:na]
        at de.neuland.pug4j.spring.view.PugView.doRender(PugView.java:57) ~[spring-pug4j-3.2.0.jar:na]
        at de.neuland.pug4j.spring.view.PugView.renderMergedTemplateModel(PugView.java:28) ~[spring-pug4j-3.2.0.jar:na]
        at org.springframework.web.servlet.view.AbstractTemplateView.renderMergedOutputModel(AbstractTemplateView.java:181) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:314) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1431) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1167) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1106) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:979) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564) ~[tomcat-embed-core-10.1.18.jar:6.0]
        at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.1.3.jar:6.1.3]
        at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.18.jar:6.0]
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) ~[tomcat-embed-websocket-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-6.1.3.jar:6.1.3]
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.1.3.jar:6.1.3]
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-6.1.3.jar:6.1.3]
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.1.3.jar:6.1.3]
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-6.1.3.jar:6.1.3]
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.1.3.jar:6.1.3]
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:167) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:115) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:340) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:391) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:896) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1744) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-10.1.18.jar:10.1.18]
        at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]

The root cause of the issue is that doctypeLine in PugTemplate is updated from multiple threads during rendering a template. The fix creates a local copy of PugTemplate for each call to render a template and populates the data from the original PugTemplate.