This changeset fixes a PHP notice appearing when a template part is created in the site editor. It also adds a unit test case to cover
`WP_REST_Templates_Controller::prepare_item_for_database`.
Props wildworks, dunhakdis, Rahmohn, oglekler, audrasjb, mukesh27, costdev, dunhakdis.
Fixes#57851.
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git-svn-id: http://core.svn.wordpress.org/trunk@55760 1a063a9b-81f0-0310-95a4-ce76da25c4cd