LDAP: fix db update for Postgres

apps/user_ldap/appinfo/database.xml

@@ -130,7 +130,7 @@
    </field>
 
    <index>
-    <name>ldap_group_members</name>
+    <name>ldap_group_members_index</name>
     <unique>true</unique>
     <field>
      <name>owncloudname</name>

apps/user_ldap/appinfo/update.php

@@ -34,7 +34,7 @@ $groupBE = new \OCA\user_ldap\GROUP_LDAP();
 $groupBE->setConnector($connector);
 
 foreach($objects as $object) {
-	$fetchDNSql = 'SELECT `ldap_dn`, `owncloud_name` FROM `*PREFIX*ldap_'.$object.'_mapping` WHERE `directory_uuid` = ""';
+	$fetchDNSql = 'SELECT `ldap_dn`, `owncloud_name` FROM `*PREFIX*ldap_'.$object.'_mapping` WHERE `directory_uuid` = \'\'';
 	$updateSql = 'UPDATE `*PREFIX*ldap_'.$object.'_mapping` SET `ldap_DN` = ?, `directory_uuid` = ? WHERE `ldap_dn` = ?';
 
 	$query = OCP\DB::prepare($fetchDNSql);