Dont throw exeption if querystring is null

Former-commit-id: 389ec310e9c3a0791e16ef4ad958727d9faf58bf
Former-commit-id: d66b23cae570d300d6de5a3d771c7d74e97db405

src/ImageProcessor.Web/Helpers/ImageHelpers.cs

@@ -78,7 +78,12 @@ namespace ImageProcessor.Web.Helpers
             {
                 // Test against the path minus the querystring so any other
                 // processors don't interere.
-                string trimmed = fullPath.Replace(queryString, string.Empty);
+                string trimmed = fullPath;
+                if (queryString != null)
+                {
+                    trimmed = trimmed.Replace(queryString, string.Empty);
+                }
+                
                 match = FormatRegex.Match(trimmed);
             }