Query: Check if the theme supports block-templates before calling locate_block_template() in get_query_template().

This change improves performance for classic themes by removing an unnecessary query and fixes an issue where a classic theme would show "Empty template: Index" on the frontend when an empty `(block-)templates/index.html` file exists.

Props johnbillion, ianatkins, Mamaduka, costdev, manfcarlo, dolphingg, audrasjb, madeinua, kapilpaul, rafiahmedd, SergeyBiryukov.
Fixes #54844.


git-svn-id: https://develop.svn.wordpress.org/trunk@52697 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-template.php

@@ -48,6 +48,10 @@ function _add_template_loader_filters() {
 function locate_block_template( $template, $type, array $templates ) {
 	global $_wp_current_template_content;
 
+	if ( ! current_theme_supports( 'block-templates' ) ) {
+		return $template;
+	}
+
 	if ( $template ) {
 		/*
 		 * locate_template() has found a PHP template at the path specified by $template.

tests/phpunit/tests/block-template.php

@@ -18,6 +18,7 @@ class Tests_Block_Template extends WP_UnitTestCase {
 	public function set_up() {
 		parent::set_up();
 		switch_theme( 'block-theme' );
+		do_action( 'setup_theme' );
 	}
 
 	public function tear_down() {