Fixes PHP errors if no values exp_file_dimensions

Should fix the issue where the fieldtype throws a bunch of PHP errors when there are no values in exp_file_dimensions (See https://github.com/krea/EE_Field_Pack/issues/14). Needs a 2nd set of eyes, I've not tested thoroughly...
Merge pull request #18 from nathanpitman/patch-2

Fixes PHP errors if no values exp_file_dimensions