fix: avoid NPE in LookupServletContainerInitializer (#11233) (#11238)

fixes #11218

Co-authored-by: Denis <denis@vaadin.com>

flow-server/src/main/java/com/vaadin/flow/server/startup/LookupServletContainerInitializer.java

@@ -58,6 +58,15 @@ public class LookupServletContainerInitializer
     @Override
     public void process(Set<Class<?>> classSet, ServletContext servletContext)
             throws ServletException {
+        if (classSet == null) {
+            throw new ServletException(ServletContainerInitializer.class
+                    .getSimpleName() + " is called but the "
+                    + "provided set of classes is 'null'. "
+                    + LookupInitializer.class + " always presents "
+                    + "and has to be passed to the 'onStartup' method as an argument "
+                    + "in the set of classes if the servlet container supports Servlet 3.0 specification. "
+                    + "The propject configuration is broken somehow or you are using Servlet 3.0 incompatible container.");
+        }
         if (!classSet.contains(LookupInitializer.class)) {
             // this is a specific case for OSGi (PAX web): at some point it may
             // decide to apply ServletContainerInitializers for non WAR case

flow-server/src/test/java/com/vaadin/flow/server/startup/LookupServletContainerInitializerTest.java

@@ -143,6 +143,11 @@ protected Collection<Class<?>> getServiceTypes() {
         Assert.assertEquals(ArrayList.class, collection.iterator().next());
     }
 
+    @Test(expected = ServletException.class)
+    public void process_classSetIsNull_throws() throws ServletException {
+        initializer.process(null, Mockito.mock(ServletContext.class));
+    }
+
     private Lookup mockLookup(ServletContext context, Class<?>... classes)
             throws ServletException {
         ArgumentCaptor<Lookup> lookupCapture = ArgumentCaptor