REST API: In `WP_REST_Block_Directory_Controller::get_items()`, make sure the...

REST API: In `WP_REST_Block_Directory_Controller::get_items()`, make sure the blocks data for a plugin is not empty before proceeding.

This avoids a PHP notice if the API returns a plugin with empty data for `blocks` key.

Props khag7, TwentyZeroTwo, justinahinon, TimothyBlynJacobs, dd32, SergeyBiryukov.
Merges [48842] to the 5.5 branch.
Fixes #51018.
Built from https://develop.svn.wordpress.org/branches/5.5@48849


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48611 1a063a9b-81f0-0310-95a4-ce76da25c4cd