REST API Unable to Set Parent to 0

[lucasstark]

Inside of WP_Rest_Posts_Controller::prepare_item_for_database there is a problem around the parent property.

The code checks if the parent is not empty, which, if you are trying to set the parent to 0 it will appear to be empty so the property is never setup for the database update.

// Parent.
		if ( ! empty( $schema['properties']['parent'] ) && ! empty( $request['parent'] ) ) {
			$parent = get_post( (int) $request['parent'] );

			if ( empty( $parent ) ) {
				return new WP_Error( 'rest_post_invalid_id', __( 'Invalid post parent ID.' ), array( 'status' => 400 ) );
			}

			$prepared_post->post_parent = (int) $parent->ID;
		}

[rachelbaker]

In 39289:

REST API: Allow parent property to be explicitly set to 0 when creating or updating a Post.

Props lucasstark, danielbachhuber.
Fixes #38852.