Sharing Buttons don't work when added to Homepage template

### Impacted plugin

None / Other

### Quick summary

When adding a Sharing Buttons block to the default Homepage template (i.e., Index template), and with no posts on the site, clicking the sharing buttons will open the site’s homepage in a new browser window instead of sharing the page.

https://github.com/user-attachments/assets/26900d76-d7d7-4522-84a8-100cc4f6a4c0

Reported in 9611351-zd-a8c

### Steps to reproduce

1. Make sure your site has a block theme active and the site has no posts.
2. Set homepage to display 'Your latest posts' under Settings > Reading
3. Add the [Sharing Buttons ](https://wordpress.com/support/wordpress-editor/blocks/sharing-buttons-block/) block to the Index template via site editor.
4. Click on the sharing buttons from the Homepage. Each button will open a new window to the Homepage instead of sharing it to the chosen platform.

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Moderate

### What other impact(s) does this issue have?

No revenue impact

### If a workaround is available, please outline it here.

The workaround is to set a static Homepage and add the Sharing Buttons block to the static page.

### Platform (Simple and/or Atomic)

Atomic, Simple

Start by reproducing the issue with a block theme, no posts, the Index template, and the Sharing Buttons block, then inspect how the buttons determine the page URL in that empty-homepage state. Done means each button opens the selected sharing service with the intended page rather than the site's homepage.

Written by the indexing model from the issue text.