Fix PHP warning for real (#7206)

plugins/markasjunk/markasjunk.php

@@ -254,10 +254,12 @@ class markasjunk extends rcube_plugin
         $a_uids = array();
 
         foreach ($messageset as $mbox => $uids) {
+            if (is_array($uids)) {
                 foreach ($uids as $uid) {
                     $a_uids[] = $multifolder ? $uid . '-' . $mbox : $uid;
                 }
             }
+        }
 
         return $a_uids;
     }