\RestfulEntityBase::getImageUris works only for single value image

If the field is multiple then we get a notice, as the $file_array is actually an array of arrays.
We should probably add a wrapper method ``\RestfulEntityBase::getImageUrisMultiple`` that calls ``\RestfulEntityBase::getImageUris``
Maybe ``isArrayNumeric`` could be replaced by a simpler ``!empty($value['fid'])`` in ``imageProcess()``?