Provide option to only insert required elements for code action cvc_complex_type_2_4_b

[JessicaJHee]

Fixes #1255

Signed-off-by: Jessica He jhe@redhat.com

[angelozerr]

@JessicaJHee let's start supporting optional element. For optional attribute, please create an issue for that.

[datho7561]

Works well!

[rgrunber]

@datho7561 , @JessicaJHee

[Error - 9:44:43 a.m.] Aug. 17, 2022 09:44:43 org.eclipse.lemminx.services.XMLCodeActions doCodeActions()
Message: Error while processing code actions for the participant 'org.eclipse.le
mminx.extensions.contentmodel.participants.ContentModelCodeActionParticipant'.
java.lang.NullPointerException: Cannot invoke "java.lang.Boolean.booleanValue()" because the return value of "java.util.Map.get(Object)" is null
    at org.eclipse.lemminx.extensions.xsd.contentmodel.CMXSDElementDeclaration.isOptional(CMXSDElementDeclaration.java:338)
    at org.eclipse.lemminx.extensions.contentmodel.utils.XMLGenerator.isGenerateChild(XMLGenerator.java:405)
    at org.eclipse.lemminx.extensions.contentmodel.utils.XMLGenerator.generate(XMLGenerator.java:150)
    at org.eclipse.lemminx.extensions.contentmodel.utils.XMLGenerator.generate(XMLGenerator.java:124)
    at org.eclipse.lemminx.extensions.contentmodel.utils.XMLGenerator.generate(XMLGenerator.java:107)
    at org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.cvc_complex_type_2_4_bCodeAction.doCodeAction(cvc_complex_type_2_4_bCodeAction.java:99)
    at org.eclipse.lemminx.extensions.contentmodel.participants.ContentModelCodeActionParticipant.doCodeAction(ContentModelCodeActionParticipant.java:51)
    at org.eclipse.lemminx.services.XMLCodeActions.doCodeActions(XMLCodeActions.java:60)
    at org.eclipse.lemminx.services.XMLLanguageService.doCodeActions(XMLLanguageService.java:247)
    at org.eclipse.lemminx.XMLTextDocumentService.lambda$codeAction$23(XMLTextDocumentService.java:464)
    at org.eclipse.lemminx.commons.ModelTextDocuments.lambda$computeModelAsync$0(ModelTextDocuments.java:118)
    at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:646)
    at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:483)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
    at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
    at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
    at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
    at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)