Improve UX for adding images to a gallery block from the media library

[dhruvikpatel18]

What?

Closes #48417

Improves the UX for adding images to a Gallery block when there is only a single image.
This PR ensures that a placeholder (empty image block) is always available after inserting the first image, making it easier to continue adding more without extra clicks.

Why?

Currently, when a Gallery block has only one image, it is not obvious how to add additional images from the media library.
Users must:

1. Select the existing image,
2. Select the gallery via the small toolbar icon,
3. Click "Add",
4. Open the media library,
5. Select more images,
6. Click "Add to gallery",
7. Finally, click "Update gallery".

This creates unnecessary friction. The “+” appender only becomes visible once the gallery has 2 or more images.
This PR reduces that friction by always showing an empty Image block placeholder immediately after the first image is added.

Testing Instructions

1. Create a new post or page.
2. Insert a Gallery block.
3. Add a single image from the media library.
   • Expected: An empty image placeholder is automatically appended after the first image.
4. Add additional images — the appender and placeholder should behave as before.
5. Remove images back down to one — the placeholder remains available to add another.
6. Confirm drag-and-drop still works as expected.

Screenshots or screencast

Gallery.Block.mov

[im3dabasia]

This does seem like a good addition, similar to the Buttons block, and other design-related blocks such as Columns, Grid, and Group, which have an appender. The gallery block currently lacks one. so adding it would improve consistency.

If there’s enough consensus, should we raise a separate follow-up PR to address this?

[dhruvikpatel18]

Thanks for the suggestion, I have raised separate follow-up PR to address above: #71236

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: dhruvikpatel18 <[email protected]>
Co-authored-by: im3dabasia <[email protected]>
Co-authored-by: hanneslsm <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.