Fix null handling in signature provider

Author: jjonescz

src/App/wwwroot/js/BlazorMonacoInterop.js

@@ -174,11 +174,12 @@ export function registerSignatureHelpProvider(language, hoverProvider) {
                 throw new Error('busy');
             }
 
-            if (result === '') {
+            const parsed = JSON.parse(result);
+
+            if (parsed === null) {
                 return null;
             }
 
-            const parsed = JSON.parse(result);
             return {
                 value: parsed,
                 dispose: () => { }, // Currently not used.

src/Compiler/LanguageServices.cs

@@ -528,7 +528,7 @@ static string addPrefix(string? prefix, string nested)
     {
         if (!TryGetDocument(modelUri, out var document))
         {
-            return "";
+            return "null";
         }
 
         var sw = Stopwatch.StartNew();