Revert PHP types in qtranxf_checkCanonical (#1314)

The main purpose is to filter `redirect_canonical` (WP hook).
The WP documentation is not precise regarding the types so we have
to relax the types check. Check first if the redirect URL is false.

The second parameter is the requested URL but it is ignored.
See #1320, wrong usage with Yoast. The second parameter has a
different meaning with Yoast. Removing the types will work but the
Yoast hook should be rewritten with a different function.

Rewrite the function to handle the default language in convertURL.

src/frontend.php

@@ -815,12 +815,19 @@ function qtranxf_updated_usermeta( int $meta_id, int $object_id, string $meta_ke
     qtranxf_cache_delete_metadata( 'user', $object_id );
 }
 
-// TODO check API with Yoast, seems it's only 1 parameter?
-function qtranxf_checkCanonical( string $redirect_url, string $requested_url ): string {
-    global $q_config;
-    $lang = $q_config['language'];
+/**
+ * Hook for redirect_canonical.
+ *
+ * @param string|false $redirect_url Attention! WordPress documents it as string, but it can be false.
+ * @param mixed $requested_url Attention! WordPress documents it as string, but it can be array or null.
+ *
+ */
+function qtranxf_checkCanonical( $redirect_url, $requested_url ): string {
+    if ( $redirect_url === false ) {
+        return false;
+    }
     // fix canonical conflicts with language urls
-    return qtranxf_convertURL( $redirect_url, $lang );
+    return qtranxf_convertURL( $redirect_url );
 }
 
 /**