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.
Reviewed by azaozz, audrasjb.
Merges [56422] to the 6.3 branch.
Fixes #58823.

 --Cette ligne, et les suivantes ci-dessous, seront 
ignorées--

_M   .
M    src/wp-includes/class-wp-classic-to-block-menu-converter.php
M    src/wp-includes/class-wp-navigation-fallback.php
M    tests/phpunit/tests/editor/classic-to-block-menu-converter.php

Built from https://develop.svn.wordpress.org/branches/6.3@56426


git-svn-id: http://core.svn.wordpress.org/branches/6.3@55938 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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 @@ class WP_Classic_To_Block_Menu_Converter {
 		$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.

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

@@ -105,6 +105,10 @@ class WP_Navigation_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.' ) );
 		}

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.3.1-alpha-56425';
+$wp_version = '6.3.1-alpha-56426';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.