Style engine: wrap array_merge in conditionals to prevent unnecessary merging

[ramonjd]

Backport changes from Core patch: WordPress/wordpress-develop#7670

Wrap array_merge in a conditional to prevent unnecessary firing.

Props to @mukeshpanchal27

Testing

CI tests should pass

[github-actions]

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[github-actions]

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[github-actions]

Flaky tests detected in 3d3a554.
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/11624372578
📝 Reported issues:

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

[aaronrobertshaw]

LGTM 🚀

✅ Code changes match core PR
✅ Tests pass locally
✅ Smoke tested style engine manually and didn't spot any issues

[ramonjd]

Thanks for the thorough testing!

[joemcgill]

Applied to core in https://core.trac.wordpress.org/changeset/59442