Do not replace html if response is empty

If the form has a file input, then the jQuery Form plugin won't handle correctly the response status and will enter in the success callback even if there is an error (see http://stackoverflow.com/questions/3995355/jquery-form-plugin-no-error-handling/7019723#7019723)

This fix checks the response length before replacing the html with a potential empty response.
Merge pull request #244 from EmmanuelVella/patch-6

Do not replace html if response is empty