Fix EZP-23207: image alt text not modified if file is not set.

JIRA: https://jira.ez.no/browse/EZP-23207

This fixes a regression in EZP-22615 , that causes an image attribute's alt text to not be updated if the file is not modified as well.
Modifying alternative text should make it so, I believe.
Actually, should the file always exist (if it's not being uploaded)?