Fix undefined array key warning in Live Search when no category is selected

[SAY-5]

Closes #2849.

Summary

When a Live Search block restricts results to post but no category is selected, otterSearchQuery['cat'] is never written to the block attributes, so render_blocks() accesses a missing key and PHP 8 emits Undefined array key "cat". Guarded the read in class-live-search.php with ?? '' so the absent key falls back to an empty data-cat attribute, matching how the surrounding post_type access is already protected.

Screenshots

Test instructions

1. Add a Search block and enable Live Search.
2. Set "Search in" to post and leave Post Category empty.
3. View the page on the frontend; no PHP warning should appear and search works with no category filter.

Added tests/test-live-search-render.php, which activates a license, renders a post-only Live Search block without a cat key, and asserts the output contains data-cat="". It fails on the warning before the fix and passes after.

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Checklist before the final review

• Included E2E or unit tests for the changes in this PR.
• Visual elements are not affected by independent changes.
• It is at least compatible with the minimum WordPress version.
• It loads additional script in frontend only if it is required.
• Does not impact the Core Web Vitals.
• In case of deprecation, old blocks are safely migrated.
• It is usable in Widgets and FSE.
• Copy/Paste is working if the attributes are modified.
• PR is following the best practices