QuickEdit: prevent site-editor from crashing when slug is not an object

[oandregal]

Related #66599 #66559

What?

This fixes a hidden bug: in some situations, the data received by slug is not an object.

How?

We've covered against that in the view, but didn't in the edit control. I've moved the check in the view to within the getSlug util, so it covers all the cases.

Testing Instructions

• Modify this line to type: 'regular'.
• Go to Site Editor > Pages, switch to table and open the QuickEdit experiment.
• Verify that it loads:

[github-actions]

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Co-authored-by: oandregal <oandregal@git.wordpress.org>
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[gigitux]

LGTM! Thanks for this fix! We need to revisit how loading state is handled #66599

[github-actions]

Flaky tests detected in be60beb.
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/12160720026
📝 Reported issues:

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