UserMetaManager: check metadata is not empty.

(cherry picked from commit ac99bf4)

Author: cdujeu

core/src/plugins/meta.user/class.UserMetaManager.php

@@ -306,6 +306,7 @@ public function extractMeta(&$ajxpNode, $contextNode = false, $details = false)
             // @todo : Should be UTF8-IZED at output only !!??
             // array_map(array("SystemTextEncoding", "toUTF8"), $metadata);
         }
+        if(empty($metadata)) $metadata = array();
         $ajxpNode->mergeMetadata($metadata);
 
     }