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.
Built from https://develop.svn.wordpress.org/trunk@56422
git-svn-id: http://core.svn.wordpress.org/trunk@55934 1a063a9b-81f0-0310-95a4-ce76da25c4cd
