Fix merging list with null

This fixes some cases observed with the debugger where we end up merging
a non empty list with null. The result is then null and the looping over
the items would then end.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>

apps/user_ldap/lib/Group_LDAP.php

@@ -24,6 +24,7 @@
  * @author Victor Dubiniuk <dubiniuk@owncloud.com>
  * @author Vinicius Cubas Brand <vinicius@eita.org.br>
  * @author Xuanwo <xuanwo@yunify.com>
+ * @author Carl Schwan <carl@carlschwan.eu>
  *
  * @license AGPL-3.0
  *
@@ -376,7 +377,7 @@ private function processListFromWalkingNestedGroups(array &$list, array &$seen,
 			$fetched = $this->access->connection->getFromCache($cacheKey);
 			if ($fetched === null) {
 				$fetched = $fetcher($recordDN);
-				$fetched = $this->access->connection->writeToCache($cacheKey, $fetched);
+				$this->access->connection->writeToCache($cacheKey, $fetched);
 			}
 			$list = array_merge($list, $fetched);
 			if (!isset($seen[$recordDN]) || is_bool($seen[$recordDN]) && is_array($record)) {