ldap: check array for emptiness, not nullness

apps/user_ldap/group_ldap.php

@@ -186,7 +186,7 @@ class OC_GROUP_LDAP extends OC_Group_Backend {
 		if(!$this->configured) {
 			return array();
 		}
-		if(is_null($this->_groups)) {
+		if(empty($this->_groups)) {
 			$ldap_groups = OC_LDAP::fetchListOfGroups($this->ldapGroupFilter, array(OC_LDAP::conf('ldapGroupDisplayName'), 'dn'));
 			$this->_groups = OC_LDAP::ownCloudGroupNames($ldap_groups);
 		}