Fix incorrect return value in StaticImportFavoritesCompletionInvoker.

[rgrunber]

• Helper method createNewCompilationUnit should always return an array.

@robstryker adopted the latest 4.31-I-builds at eclipse-jdtls/eclipse.jdt.ls#2947, and they uncovered a regression in the organize imports logic : https://ci.eclipse.org/ls/job/jdt-ls-pr/4816/ when triggered in a module file.

java.lang.NullPointerException: Cannot load from object array because "res" is null
	at org.eclipse.jdt.internal.corext.util.StaticImportFavoritesCompletionInvoker.getStaticImportFavorites(StaticImportFavoritesCompletionInvoker.java:54)
	at org.eclipse.jdt.core.manipulation.OrganizeImportsOperation.addStaticImports(OrganizeImportsOperation.java:722)
	at org.eclipse.jdt.core.manipulation.OrganizeImportsOperation.createTextEdit(OrganizeImportsOperation.java:621)
	at org.eclipse.jdt.ls.core.internal.handlers.OrganizeImportsHandler.organizeImports(OrganizeImportsHandler.java:98)
	at org.eclipse.jdt.ls.core.internal.handlers.OrganizeImportsHandler$1.addEdits(OrganizeImportsHandler.java:142)
	at org.eclipse.jdt.core.manipulation.CUCorrectionProposalCore.createTextChange(CUCorrectionProposalCore.java:188)
	at org.eclipse.jdt.core.manipulation.CUCorrectionProposalCore.createChange(CUCorrectionProposalCore.java:195)
	at org.eclipse.jdt.core.manipulation.ChangeCorrectionProposalCore.getChange(ChangeCorrectionProposalCore.java:152)
	at org.eclipse.jdt.core.manipulation.CUCorrectionProposalCore.getTextChange(CUCorrectionProposalCore.java:206)

Introduced by #927 (in 4.31-I-builds).

Before:

eclipse.jdt.ui/org.eclipse.jdt.core.manipulation/core extension/org/eclipse/jdt/internal/corext/util/JavaModelUtil.java

Lines 1327 to 1331 in 137f84f

	StringBuilder dummyCU= new StringBuilder();
	String packName= cu.getParent().getElementName();
	IType type= cu.findPrimaryType();
	if (type == null)
	return new String[0];

After:

eclipse.jdt.ui/org.eclipse.jdt.core.manipulation/core extension/org/eclipse/jdt/internal/corext/util/StaticImportFavoritesCompletionInvoker.java

Lines 84 to 88 in d04cefe

	StringBuilder dummyCU= new StringBuilder();
	String packName= fCu.getParent().getElementName();
	IType type= fCu.findPrimaryType();
	if (type == null)
	return null;

The fix is straightforward.

Update: I've added a testcase. Given that the PR that introduced the regression passed when merged, it's safe to say no tests are triggering "Organize Imports" in module files, so I've added the one we use in JDT-LS.

[robstryker]

Patch looks clean to me. Original PR seems harder to parse but the fix looks obvious with clear and concise solution.