Post: Ensure get_post_custom_values() non-array returns grace fully.

[liaisontw]

Description
This PR improves the robustness of get_post_custom_values() by adding an explicit array check before accessing the data.

Currently, get_post_custom_values() assumes that get_post_custom() always returns an array. However, in certain scenarios (such as an invalid post_id or when no global post is set), get_post_custom() can return false. Attempting to access an array offset on a boolean value triggers a "Cannot use a scalar value as an array" warning in PHP 8.2+.

Changes included in this PR:

Array Validation: Added an is_array() check in get_post_custom_values() to safely handle non-array returns from get_post_custom().

Consistency: Aligns the behavior of get_post_custom_values() with get_post_custom_keys(), ensuring both return early when data is unavailable.

Unit Tests: Introduced a new test file tests/phpunit/tests/post/getPostCustom.php covering happy paths, invalid IDs, and empty key scenarios to prevent future regressions.

Trac ticket: https://core.trac.wordpress.org/ticket/65044

Use of AI Tools
AI assistance: Yes
Tool(s): Gemini
Model(s): Gemini 3 Flash
Used for: Structuring the PHPUnit test suite and drafting the technical justification for the PR. I have personally verified the behavior against PHP 8.2 and ensured the logic aligns with WordPress Core’s historical handling of post meta.

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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