AdminThumbnail: Use source from generator if image_field is a function

[autoantwort]

My main motivation was this example from the readme: https://github.com/matthewwithanm/django-imagekit?tab=readme-ov-file#admin (To use specs defined outside of models:)

If you use this example code and click on the thumbnail image in the admin interface, the thumbnail image will open and not the original image. With this PR the original image will be opened.