Make FocusedJavaAllProposalComputer more robust

[ruspl-afed]

During attempt to complete invalid Java code there is a Mylyn failure reported:

The 'org.eclipse.mylyn.java.ui.javaAllCompletionProposalComputer' proposal computer from the 'org.eclipse.mylyn.java.ui' plug-in did not complete normally. The extension has thrown a runtime exception.

Perhaps this is pure JDT problem, but we can try to be more robust on Mylyn side

java.lang.NullPointerException: Cannot invoke "org.eclipse.jdt.internal.compiler.impl.Constant.typeID()" because "this.constant" is null
	at org.eclipse.jdt.internal.compiler.ast.Expression.computeConversion(Expression.java:707)
	at org.eclipse.jdt.internal.compiler.ast.SingleNameReference.computeConversion(SingleNameReference.java:320)
	at org.eclipse.jdt.internal.compiler.ast.BinaryExpression.resolveType(BinaryExpression.java:1921)
	at org.eclipse.jdt.internal.compiler.ast.CombinedBinaryExpression.resolveType(CombinedBinaryExpression.java:398)
	at org.eclipse.jdt.internal.compiler.ast.MemberValuePair.resolveTypeExpecting(MemberValuePair.java:77)
	at org.eclipse.jdt.internal.compiler.ast.Annotation.resolveType(Annotation.java:956)
	at org.eclipse.jdt.internal.compiler.ast.ASTNode.resolveAnnotations(ASTNode.java:970)
	at org.eclipse.jdt.internal.compiler.ast.ASTNode.resolveAnnotations(ASTNode.java:800)
	at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.getAnnotationTagBits(SourceTypeBinding.java:1642)
	at org.eclipse.jdt.internal.compiler.ast.TypeDeclaration.updateSupertypesWithAnnotations(TypeDeclaration.java:1886)
	at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.integrateAnnotationsInHierarchy(CompilationUnitScope.java:402)
	at org.eclipse.jdt.internal.compiler.lookup.LookupEnvironment$CompleteTypeBindingsSteps.perform(LookupEnvironment.java:173)
	at org.eclipse.jdt.internal.compiler.lookup.LookupEnvironment.completeTypeBindings(LookupEnvironment.java:605)
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.complete(CompletionEngine.java:2329)
	at org.eclipse.jdt.internal.core.Openable.codeComplete(Openable.java:136)
	at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:367)
	at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:357)
	at org.eclipse.jdt.internal.ui.text.java.JavaCompletionProposalComputer.internalComputeCompletionProposals(JavaCompletionProposalComputer.java:256)
	at org.eclipse.jdt.internal.ui.text.java.JavaCompletionProposalComputer.computeCompletionProposals(JavaCompletionProposalComputer.java:218)
	at org.eclipse.jdt.internal.ui.text.java.JavaTypeCompletionProposalComputer.computeCompletionProposals(JavaTypeCompletionProposalComputer.java:65)
	at org.eclipse.mylyn.internal.java.ui.editor.FocusedJavaAllProposalComputer.computeCompletionProposals(FocusedJavaAllProposalComputer.java:42)
	at org.eclipse.jdt.internal.ui.text.java.CompletionProposalComputerDescriptor.computeCompletionProposals(CompletionProposalComputerDescriptor.java:348)
	at org.eclipse.jdt.internal.ui.text.java.CompletionProposalCategory.computeCompletionProposals(CompletionProposalCategory.java:340)
	at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.collectProposals(ContentAssistProcessor.java:333)
	at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.computeCompletionProposals(ContentAssistProcessor.java:289)
	at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$10(AsyncCompletionProposalPopup.java:378)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$9(AsyncCompletionProposalPopup.java:377)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
	at java.base/java.util.concurrent.ForkJoinPool$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)