fix: do not index WP_Error when polling optimized images

[selul]

Summary

• Dashboard optimized-images polling called get_optimized_images(), which is documented to return array|bool|WP_Error, then indexed $images['list'] with no type check. Transport failures and non-200 API error payloads therefore fatal with Cannot use object of type WP_Error as array (inc/rest.php:712).
• Treat WP_Error, non-arrays, and missing/empty lists the same as “no images”: return a successful REST payload with an empty list so the dashboard widget degrades instead of crashing.
• Covers the 4.2.11 telemetry in Optimized-images polling crashes on WP_Error responses #1125 (3 sites). This is a standing type hole, not an update/OPcache race.

Fixes #1125

Test plan

• composer phpunit -- --filter=Test_Poll_Optimized_Images (transport WP_Error, API error payload, empty body, missing/empty list, successful list)
• PHPCS on inc/rest.php, PHPStan
• In the dashboard, with Optimole disconnected from the stats API / blocked HTTP, Last optimized images should stay empty instead of a REST 500
• With a working connection, the last-optimized-images widget still lists images

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