Add null check for 'fCollapsedTypes' variable to avoid NPE when invoking completion in DEBUG CONSOLE

[testforstephen]

With the pre-release version of Java language support extension installed, triggering method completion on DEBUG CONSOLE will throw NPE exception as below.

Cannot invoke "java.util.Map.getOrDefault(Object, Object)" because "this.fCollapsedTypes" is null
!STACK 0
java.lang.NullPointerException: Cannot invoke "java.util.Map.getOrDefault(Object, Object)" because "this.fCollapsedTypes" is null
	at org.eclipse.jdt.ls.core.internal.contentassist.CompletionProposalDescriptionProvider.createMethodProposalLabel(CompletionProposalDescriptionProvider.java:334)
	at org.eclipse.jdt.ls.core.internal.contentassist.CompletionProposalDescriptionProvider.updateDescription(CompletionProposalDescriptionProvider.java:745)
	at com.microsoft.java.debug.plugin.internal.CompletionProposalRequestor.toCompletionItem(CompletionProposalRequestor.java:165)
	at com.microsoft.java.debug.plugin.internal.CompletionProposalRequestor.getCompletionItems(CompletionProposalRequestor.java:147)
	at com.microsoft.java.debug.plugin.internal.CompletionsProvider.codeComplete(CompletionsProvider.java:75)
	at com.microsoft.java.debug.core.adapter.handler.CompletionsHandler.lambda$handle$0(CompletionsHandler.java:68)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(Unknown Source)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)

The issue occurs because the Java Debugger uses the old constructor CompletionProposalDescriptionProvider(CompletionContext context) to initialize the DescriptionProvider. This constructor does not initialize the new field fCollapsedTypes, resulting in a NullPointerException when fCollapsedTypes is later referenced. It is important to ensure that when introducing new fields in the code, they do not break any existing usage.

[fbricon]

if the maps are read-only, then Collections.emptyMap() would be better.

[testforstephen]

I change the approach to add null check to avoid NPE, that's a more straightforward fix.