hotfix Validator\File\MimeType triggering warning

an empty `$_FILES` array would be passed to the validator under
some circumstances, e.g. trying to upload a file that exceeds the
php.ini `upload_max_filesize` value.

this PR prevents the warning that is triggered by `finfo_file()`
when passing an empty string as second parameter, harming the correct
behaviour of the validator (set an error message and return false).