Media image structures fatal on a field that does not reference media

Problem/Motivation

MyrestImageBuilder::getStructureMediaImages() calls getSource() on every entity the field references. When the field references files — an image field under the same name, which is an ordinary way to model a picture — the response fatals with Call to undefined method ...File::getSource(). Its neighbour getStructureMediaImage() checks instanceof MediaInterface first and does not.

A module meant to work on the model a site actually has must not fatal because a site chose a file field where another chose media.

Proposed resolution

• Skip anything that is not media, the way the single-image method does.
• Better: answer an image field with the same structure as a media image, so that both modellings produce the key rather than one of them producing nothing.
• A kernel test with both kinds of field under the same name.

Remaining tasks

Everything.