Check if cache is present with isset

Otherwise we get false for empty array

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

apps/user_ldap/lib/Group_LDAP.php

@@ -797,7 +797,7 @@ private function getGroupsByMember(string $dn, array &$seen = []): array {
 		}
 		$seen[$dn] = true;
 
-		if ($this->cachedGroupsByMember[$dn]) {
+		if (isset($this->cachedGroupsByMember[$dn])) {
 			return $this->cachedGroupsByMember[$dn];
 		}