Check for XML before overwriting

Thank you for putting this extension together - very helpful! 

I did run into an issue using it in combination with the Conditionalizer extension. Subsequent queries would overwrite Conditionalizer's attempt to stop queries, essentially removing Conditionalizer from the equation. It is easily fixed by checking whether `$context['xml']` is empty before proceeding (https://github.com/ahwayakchih/conditionalizer/issues/8#issuecomment-25923310). Unless you forsee other problems from this small change, I think it would be a benefit for these two extensions to be able to work together.

Thanks.