Do not return an error on mailinglist listing if none exists

We usually return the empty array, to avoid the need for specific error
handling

src/mailboxdb.js

@@ -146,7 +146,6 @@ function listGroups(domain, callback) {
 
     database.query('SELECT ' + MAILBOX_FIELDS + ' FROM mailboxes WHERE domain = ? AND ownerType = ? AND aliasTarget IS NULL', [ domain, exports.TYPE_GROUP ], function (error, results) {
         if (error) return callback(new DatabaseError(DatabaseError.INTERNAL_ERROR, error));
-        if (results.length === 0) return callback(new DatabaseError(DatabaseError.NOT_FOUND));
 
         callback(null, results);
     });