fix(newspack-popups): cache empty popups_for_post() results

[jhalitschke]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Re-opened against the monorepo — originally submitted as Automattic/newspack-popups#1563, auto-closed since that repo is now a read-only mirror.

Problem

Newspack_Popups_Inserter::popups_for_post() (plugins/newspack-popups/includes/class-newspack-popups-inserter.php) memoizes its result in self::$popups, guarded by:

protected static $popups = [];

public static function popups_for_post() {
	if ( ! empty( self::$popups ) ) {
		return self::$popups;
	}
	...
	self::$popups = $popups_to_display; // may be []
	return $popups_to_display;
}

empty( [] ) is always true, so whenever no campaign is eligible for the current request, the cache is written but never read back as a hit. Every subsequent call in the same request re-runs assess_has_disabled_popups(), parse_view_as(), Newspack_Popups_Model::retrieve_eligible_popups() (a WP_Query) and a should_display() pass over every campaign.

popups_for_post() is called from insert_popups_in_content(), insert_popups_after_header(), insert_before_header() and insert_inline_prompt_in_archive_pages(), so on an archive page this repeats once per post instead of once per request. On a site where no campaigns match the request context (e.g. all campaigns are scoped to a segment/view that excludes the current visitor), this turns into 40-50+ redundant query + evaluation passes on a single archive page load.

Fix

Use null instead of [] as the "not yet computed" sentinel, since an empty array is a legitimate, cacheable result:

protected static $popups = null;

public static function popups_for_post() {
	if ( null !== self::$popups ) {
		return self::$popups;
	}
	...
}

No other change in behavior: the IS_TEST_ENV branch already skips writing the cache, so tests are unaffected either way, and nothing else reads Newspack_Popups_Inserter::$popups directly (the data-api class has its own unrelated $popups property).

Closes # .

How to test the changes in this Pull Request:

1. php -l on the changed file
2. Verify prompts still render on posts/pages that have eligible campaigns
3. Verify archive pages with zero eligible campaigns no longer re-run retrieve_eligible_popups() per post (checked via query count / profiling)

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

This PR aims to reduce redundant work in newspack-popups by ensuring Newspack_Popups_Inserter::popups_for_post() memoization treats an empty result as a valid cache hit (using null as the “not computed yet” sentinel).

Changes:

• Change Newspack_Popups_Inserter::$popups default from [] to null and update its docblock to reflect the new sentinel behavior.
• Update the memoization guard in popups_for_post() from ! empty( self::$popups ) to null !== self::$popups so empty results are cached/hit.