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

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>