fix(LDAP): ensure stored groups are formatted as simple list

With array_unique it is possible that the keys are not in sequential order
but have gaps. json_encode then would store them as associative array,
which later on json_decode would result in a stdClass by default. This is
unexpected and would also contradict the return type hint.

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

apps/user_ldap/lib/Group_LDAP.php

@@ -685,7 +685,7 @@ private function isUserOnLDAP(string $uid): bool {
 
 	protected function getCachedGroupsForUserId(string $uid): array {
 		$groupStr = $this->config->getUserValue($uid, 'user_ldap', 'cached-group-memberships-' . $this->access->connection->getConfigPrefix(), '[]');
-		return json_decode($groupStr) ?? [];
+		return json_decode($groupStr, true) ?? [];
 	}
 
 	/**
@@ -838,7 +838,7 @@ public function getUserGroups($uid): array {
 			return $groups;
 		}
 
-		$groups = array_unique($groups, SORT_LOCALE_STRING);
+		$groups = array_values(array_unique($groups, SORT_LOCALE_STRING));
 		$this->access->connection->writeToCache($cacheKey, $groups);
 
 		$groupStr = \json_encode($groups);