Missing validation on $data args

Occasionally we'll get illegal offset errors because `$data` isn't set in [this function](https://github.com/alleyinteractive/photonfill/blob/master/php/class-photonfill-transform.php#L102), specifically, [these lines](https://github.com/alleyinteractive/photonfill/blob/master/php/class-photonfill-transform.php#L109-L112).

This occurs when `$data` is a string instead of an array.