Prevent PHP notice from block-supports elements

[carolinan]

Description

When using the Post Tags block and setting a link color, the front end gives PHP errors, when a post is untagged:
Notice: Undefined offset: 0 in C:\xampp\apps\wordpress\htdocs\wp-content\plugins\gutenberg\lib\block-supports\elements.php on line 47
This PR adds an early return if there is no block to add the style to.
Closes #35769

How has this been tested?

1. Add a post tag block to a post without tags, and add a link color.
2. Add a few other blocks with link color options.
3. View the front and confirm that there is no PHP notice, and that the link colors display correctly.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[Mamaduka]

This looks great! Thanks, @carolinan.