\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``
I like the `isArrayNumeric` because it is a general, yet simple, solution. Looking for a specific key is bound to a specific use case. What if we need to act on lists of nodes later on, for instance.