Fix PHP warning when user not logged in

Warning: Trying to access array offset on value of type null in
./dokuwiki/move/action/rename.php on line 42

$USERINFO is null when user is not logged in.

Regression introduced by 0af31bb (#246).

Fixes #256, #255

action/rename.php

@@ -39,7 +39,11 @@ public function handle_init() {
         global $USERINFO;
 
         $JSINFO['move_renameokay'] = $this->renameOkay($INFO['id']);
-        $JSINFO['move_allowrename'] = auth_isMember($this->getConf('allowrename'), $INPUT->server->str('REMOTE_USER'), (array) $USERINFO['grps']);
+        $JSINFO['move_allowrename'] = auth_isMember(
+            $this->getConf('allowrename'),
+            $INPUT->server->str('REMOTE_USER'),
+            $USERINFO['grps'] ?? []
+        );
     }
 
     /**