Fixed check for message.

Former-commit-id: 60177850e28645374a56ba7da941663e46f5e872
Former-commit-id: 50fd3960da2e9396a51dd299da7c5f287b3624a1
Former-commit-id: 0d0a319ea55a40dcc451913cff14fccfa8df5d26

src/ImageProcessorCore/Common/Helpers/Guard.cs

@@ -26,7 +26,7 @@ namespace ImageProcessorCore
         {
             if (target == null)
             {
-                if (string.IsNullOrWhiteSpace(message))
+                if (!string.IsNullOrWhiteSpace(message))
                 {
                     throw new ArgumentNullException(parameterName, message);
                 }