Update ApiErrorMessageSafe to return null for empty and whitespace strings (#1540)

* return null if ApiError.Message is empty or whitespace

* Uncomment test, which now passes

Octokit/Exceptions/ApiException.cs

@@ -178,7 +178,12 @@ namespace Octokit
         {
             get
             {
-                return ApiError != null ? ApiError.Message : null;
+                if (ApiError != null && !string.IsNullOrWhiteSpace(ApiError.Message))
+                {
+                    return ApiError.Message;
+                }
+
+                return null;
             }
         }