[Bug][API-SERVER] An exception will be thrown directly when exporting workflow definition that includes Mysql Task

[echohlne]

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Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior, for example:

1. create a process definition
2. create a mysql query task
3. export the process definition, we would get a empty json, and the log shows below:

[ERROR] 2021-07-20 00:12:26.280 org.apache.dolphinscheduler.api.controller.ProcessDefinitionController:[640] - batch export process definition by ids error
java.lang.ClassCastException: com.fasterxml.jackson.databind.node.POJONode cannot be cast to com.fasterxml.jackson.databind.node.ObjectNode
	at org.apache.dolphinscheduler.api.utils.exportprocess.DataSourceParam.addExportSpecialParam(DataSourceParam.java:51)
	at org.apache.dolphinscheduler.api.service.impl.ProcessDefinitionServiceImpl.addExportTaskNodeSpecialParam(ProcessDefinitionServiceImpl.java:728)
	at org.apache.dolphinscheduler.api.service.impl.ProcessDefinitionServiceImpl.exportProcessMetaData(ProcessDefinitionServiceImpl.java:686)
	at org.apache.dolphinscheduler.api.service.impl.ProcessDefinitionServiceImpl.getProcessDefinitionList(ProcessDefinitionServiceImpl.java:639)
	at org.apache.dolphinscheduler.api.service.impl.ProcessDefinitionServiceImpl.batchExportProcessDefinitionByIds(ProcessDefinitionServiceImpl.java:621)
	at org.apache.dolphinscheduler.api.service.impl.ProcessDefinitionServiceImpl$$FastClassBySpringCGLIB$$e8e34ed9.invoke(<generated>)
	at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:687)
	at org.apache.dolphinscheduler.api.service.impl.ProcessDefinitionServiceImpl$$EnhancerBySpringCGLIB$$f87458f2.batchExportProcessDefinitionByIds(<generated>)
	at org.apache.dolphinscheduler.api.controller.ProcessDefinitionController.batchExportProcessDefinitionByIds(ProcessDefinitionController.java:638)
	at org.apache.dolphinscheduler.api.controller.ProcessDefinitionController$$FastClassBySpringCGLIB$$dc9bf5db.invoke(<generated>)
	at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:752)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
	at org.springframework.aop.aspectj.MethodInvocationProceedingJoinPoint.proceed(MethodInvocationProceedingJoinPoint.java:88)
	at org.apache.dolphinscheduler.api.aspect.AccessLogAspect.doAround(AccessLogAspect.java:86)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)

Which version of Dolphin Scheduler:
latest dev branch

[github-actions]

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