Call success with nil if response data has length == 0.

Fixes exception when attempting to parse empty response data (i.e. HTTP Status Code 204 No Content).

AFNetworking/AFJSONRequestOperation.m

@@ -61,6 +61,10 @@
             if (failure) {
                 failure(error);
             }
+        } else if ([data length] == 0) {
+            if (success) {
+                success(nil);
+            }
         } else {
             id JSON = nil;