Editor: Fix error handling of converting classic to block menus.

Fixes the error handling for when `WP_Classic_To_Block_Menu_Converter::convert()` returns an instance of `WP_Error`. `WP_Navigation_Fallback::create_classic_menu_fallback()` now checks for `is_wp_error()` and if `true`, returns the error. And the `@return` type is updated to `string|WP_Error`.

Also includes a fix in the return type in `WP_Classic_To_Block_Menu_Converter::convert()` to return an empty string instead of an array instead, i.e. when bailing out for no menu items returned by `wp_get_nav_menu_items()`. The return type is clearly documented as a `string`.

Follow-up to [56052].

Props dlh, get_dave, antonvlasenko, hellofromTonya.
Fixes #58823.

git-svn-id: https://develop.svn.wordpress.org/trunk@56422 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-classic-to-block-menu-converter.php

@@ -20,7 +20,9 @@ class WP_Classic_To_Block_Menu_Converter {
 	 * @since 6.3.0
 	 *
 	 * @param WP_Term $menu The Menu term object of the menu to convert.
-	 * @return string the serialized and normalized parsed blocks.
+	 * @return string|WP_Error The serialized and normalized parsed blocks on success,
+	 *                         an empty string when there are no menus to convert,
+	 *                         or WP_Error on invalid menu.
 	 */
 	public static function convert( $menu ) {
 
@@ -34,7 +36,7 @@ public static function convert( $menu ) {
 		$menu_items = wp_get_nav_menu_items( $menu->term_id, array( 'update_post_term_cache' => false ) );
 
 		if ( empty( $menu_items ) ) {
-			return array();
+			return '';
 		}
 
 		// Set up the $menu_item variables.

src/wp-includes/class-wp-navigation-fallback.php

@@ -105,6 +105,10 @@ private static function create_classic_menu_fallback() {
 		// If there is a classic menu then convert it to blocks.
 		$classic_nav_menu_blocks = WP_Classic_To_Block_Menu_Converter::convert( $classic_nav_menu );
 
+		if ( is_wp_error( $classic_nav_menu_blocks ) ) {
+			return $classic_nav_menu_blocks;
+		}
+
 		if ( empty( $classic_nav_menu_blocks ) ) {
 			return new WP_Error( 'cannot_convert_classic_menu', __( 'Unable to convert Classic Menu to blocks.' ) );
 		}

tests/phpunit/tests/editor/classic-to-block-menu-converter.php

@@ -205,16 +205,14 @@ public function test_does_not_convert_menu_items_with_non_publish_status() {
 	 * @ticket 58557
 	 * @covers WP_Classic_To_Block_Menu_Converter::convert
 	 */
-	public function test_returns_empty_array_for_menus_with_no_items() {
+	public function test_returns_empty_string_for_menus_with_no_items() {
 		$menu_id = wp_create_nav_menu( 'Empty Menu' );
 
 		$classic_nav_menu = wp_get_nav_menu_object( $menu_id );
 
 		$blocks = WP_Classic_To_Block_Menu_Converter::convert( $classic_nav_menu );
 
-		$this->assertEmpty( $blocks, 'Result should be empty.' );
-
-		$this->assertIsArray( $blocks, 'Result should be empty array.' );
+		$this->assertSame( '', $blocks, 'Result should be empty string.' );
 
 		wp_delete_nav_menu( $menu_id );
 	}