feat(ui): add empty state for regional guidance ref image

[psychedelicious]

Summary

Adds empty state for reference images, like we have for control adapters.

Screen.Recording.2024-11-20.at.1.19.10.pm.mov

• Empty state for regional guidance reference images
• Empty state for global reference images

Related Issues / Discussions

offline discussion

QA Instructions

One caveat - we don't track whether or not it is the first time that you have selected an image. When you click the reset icon in the corner of the reference image image, we fall back to the empty state.

This differs from the current behaviour, where clicking the reset icon only empties the image - the rest of the ref image settings are still displayed, with the image box being a placeholder. You can see this in the video.

We'll need to make some deeper changes if we want to track whether or not this is the first time the user has selected a reference image for a given layer.

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)