Code Quality: Use the null coalescing assignment operator for array initialization

[Soean]

Description

Covering one recognizable idiom: initializing a container to an empty array before writing into it.

-if ( ! isset( $wp_meta_boxes[ $page ] ) ) {
-	$wp_meta_boxes[ $page ] = array();
-}
+$wp_meta_boxes[ $page ] ??= array();

Every one of the 58 changed sites has exactly this shape, an isset() guard whose entire body is a single = array(); assignment to the tested expression. There is no per-site judgement involved, which makes the change easy to verify: confirm the rule once, then scan.

Why this is safe

• ??= requires PHP 7.4, which is already the minimum supported version.
• ??= and ! isset() test the same condition, so the array is created in exactly the same cases.
• Only blocks whose body is a single assignment were converted. Any guard containing further statements was skipped, since ??= would let those statements run unconditionally.
• Nested initializations keep their order, so parent arrays are still created before their children:

  $wp_meta_boxes                      ??= array();
  $wp_meta_boxes[ $page ]             ??= array();
  $wp_meta_boxes[ $page ][ $context ] ??= array();

• No behavior change.

Trac ticket: https://core.trac.wordpress.org/ticket/65823

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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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