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.Tests/Http/ConnectionTests.cs

@@ -349,7 +349,7 @@ namespace Octokit.Tests.Http
                 Assert.Equal(45, exception.RetryAfterSeconds);
             }
 
-            [Fact(Skip = "Fails due to https://github.com/octokit/octokit.net/issues/1529. The message is empty but the default message isn't displayed. However, this isn't due to the introduction of AbuseException, but rather something else.")]
+            [Fact]
             public async Task ThrowsAbuseExceptionWithDefaultMessageForUnsafeAbuseResponse()
             {
                 string messageText = string.Empty;

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;
             }
         }