REST API: Prevent fatal error when a null template reaches prepare_item_for_response()

[ramonjd]

Trac ticket: https://core.trac.wordpress.org/ticket/66032

Backports the guard from WordPress/gutenberg#82374.

What

Prevents an uncaught fatal error (Attempt to assign property "content" on null) in WP_REST_Templates_Controller::prepare_item_for_response() when it receives a null template.

Why

WP_REST_Templates_Controller::update_item() passes its get_block_template() refetches to prepare_item_for_response() without checking them, both after writing an update and on its revert-to-theme path, which force-deletes the template's post before checking that a theme or plugin version of the template exists. When a refetch returns null, the method fatals reading $item->content.

What changed

• prepare_item_for_response() returns a rest_template_not_found WP_Error (404) when the template is null, instead of crashing.
• Adds a unit test that calls prepare_item_for_response( null, ... ) directly, the exact condition that fatals on trunk. Without the guard the call fatals and the run aborts, so the test can only pass through the guard.

Manual testing

The revert path is the easiest way to reach the fatal:

1. Create a template that exists only in the database: POST /wp/v2/templates with { "slug": "db-only", "title": "T", "content": "" }.
2. Send PUT /wp/v2/templates/<theme>//db-only with { "source": "theme" }.
3. On trunk this fatals with the error above; with this branch it returns 404 rest_template_not_found.

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