Add placeholder flag when image widget has empty _image array

[skosarin]

Summary

Fixes an issue where the image widget fails to display its placeholder after a previously selected image is archived, leaving the widget in a broken state with an empty _image relationship.

The fix extends sanitize to set aposPlaceholder = true before validation when the widget has an empty imageIds array. This is necessary because aposPlaceholder widgets bypass schema conversion entirely — without marking the widget as a placeholder early, the required _image field triggers an error that crashes ApostropheCMS's handleConvertErrors due to an upstream bug with numeric error paths.

What are the specific steps to test this change?

1. Run the website and log in as an admin.
2. Create a page with an image widget and select an image.
3. Archive the selected image (so the widget's _image relationship becomes empty).
4. Re-open the page editor containing the image widget.
5. Confirm the image widget displays the placeholder (and does not behave as if an image is still selected).

What kind of change does this PR introduce?

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

[boutell]

Thanks for contributing this. I see the reasoning behind it and I do see how it would be useful to editors. However this would also cause it to appear for the public, which is never intended to occur. So it wouldn't be suitable in its current form.

However, checking for req.query.aposEdit === '1' first might resolve that. I suggest you try that and see if it ensures the placeholder is displayed only when editing and not previewing or just a member of the public viewing the site.

[skosarin]

at least on my local, it behaves as desired -- no placeholder image when in preview/published, but placeholder appears on edit, and no page error occurs

[boutell]

OK fair, we will QA it directly to see if any further change is actually needed cc @BoDonkey

[boutell]

Oh excuse me, I see you did add the change! Your comment makes more sense now 👍