Fix: Don't show aria-label when its value is empty

[SantosGuillamot]

What?

Fixes an accessibility issue reported here.

Why?

aria-label shouldn't be present when its value is empty.

How?

Checking if it is empty before passing it as an extra attribute.

Testing Instructions

1. Go to a site with the default navigation block.
2. Inspect the nav element.
3. Check that aria-label is not there. Previously, it was without any value.

[github-actions]

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Co-authored-by: SantosGuillamot <santosguillamot@git.wordpress.org>
Co-authored-by: afercia <afercia@git.wordpress.org>
Co-authored-by: cbravobernal <cbravobernal@git.wordpress.org>

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[github-actions]

Flaky tests detected in 8f952e0.
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/12066203988
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[afercia]

LGTM thanks!

[cbravobernal]

LGTM