On old carburettor cars, the AC might actually reduce idle fuel consumption because the AC load reduces the engine’s idle rpm, unless there is an idle kick up solenoid to raise the engine’s idle speed a little when the AC is on. Ditto with leaving it in drive.
When you put a load on an idling engine, its rpm lowers and that reduces the intake manifold vacuum, the lower vacuum sucks less air through closed throttle and that means the carburettor mixes less fuel with the airflow.
Modern EFI cars have an adaptive idle airflow, the throttle opens slightly to offset the extra load of AC on or being in drive and so there is a measurable difference in idle fuel consumption in both cases. On my car, the idle speed actually jumps up a few rpm when the AC is on.