Cover: Avoid adding empty background image URL

[Mamaduka]

What?

This a follow-up to #49434 (comment).

Update the Cover block's render logic to return early when a post has no featured image and avoid processing the content markup.

Props @t-hamano for noticing this edge case.

Why?

There's no need to modify the Cover block's markup when a post has no featured image.

Testing Instructions

1. Open a Post or Page.
2. Add the featured image to the post from the Post panel
3. Insert a cover block
4. Enable "Fixed background" or "Repeated background"
5. Delete the featured image from the Post panel.
6. Confirm empty background-image:url() isn't rendered on front end.

Screenshots or screencast

[t-hamano]

LGTM! I think an early return makes more sense than the code I suggested 😄

[github-actions]

Flaky tests detected in 694a107.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4564723775
📝 Reported issues:

• [Flaky Test] shows changes in a template when a template part it contains is modified #42139 in /test/e2e/specs/site-editor/template-part.spec.js