Ignore empty values

Some systems (an old sqlite version) seem to store an empty string in an empty field. We do not want to check that against the current user, because it will be empty if noone is logged in. That bypasses all ACLs!

action.php

@@ -59,6 +59,7 @@ public function handleAclCheck(Doku_Event $event, $mode)
                     $col = $schema->findColumn($field);
                     if ($col && is_a($col->getType(), \dokuwiki\plugin\struct\types\User::class)) {
                         $value = $data[$field]->getValue();
+                        if (empty($value)) continue;
                         // multivalue field?
                         if (is_array($value)) {
                             $users = array_merge($users, $value);