Fix NPE in inlay hints

Author: BoykoAlex

headless-services/commons/commons-language-server/src/main/java/org/springframework/ide/vscode/commons/languageserver/util/InlayHintHandler.java

@@ -16,9 +16,10 @@
 import org.eclipse.lsp4j.Range;
 import org.eclipse.lsp4j.jsonrpc.CancelChecker;
 import org.springframework.ide.vscode.commons.util.text.TextDocument;
+import org.springframework.lang.NonNull;
 
 public interface InlayHintHandler {
 
-	List<InlayHint> handle(TextDocument doc, Range range, CancelChecker cancelChecker);
+	@NonNull List<InlayHint> handle(TextDocument doc, Range range, CancelChecker cancelChecker);
 
 }

headless-services/spring-boot-language-server/src/main/java/org/springframework/ide/vscode/boot/java/JdtInlayHintsHandler.java

@@ -56,7 +56,7 @@ public List<InlayHint> handle(TextDocument doc, Range range, CancelChecker cance
 
 	private List<InlayHint> computeInlayHints(List<JdtInlayHintsProvider> applicableInlayHintsProviders, IJavaProject jp, CompilationUnit cu, Range r, TextDocument doc) {
 		if (cu == null) {
-			return null;
+			return Collections.emptyList();
 		}
 		Collector<InlayHint> collector = new Collector<>();
 		CompositeASTVisitor visitor = new CompositeASTVisitor();