TypeImportCompletionItem.GetCompletionDescriptionAsync should not return null (#38037)

src/Features/Core/Portable/Completion/Providers/ImportCompletionProvider/TypeImportCompletionItem.cs

@@ -102,7 +102,7 @@ public static async Task<CompletionDescription> GetCompletionDescriptionAsync(Do
                 }
             }
 
-            return null;
+            return CompletionDescription.Empty;
         }
 
         private static string GetAritySuffix(int arity)