Laravel Validator exists or equal to / conditonal or

I have an input value that can be either an actual ID existing in the database or a 0 indicating special case.

In any case, if this value is sent not-empty, I need it to be validated with something like

Validator::make($data, [
    'item_id.*' => 'required|in:0[OR]exists:items_table,id',
]);

The problem is I did not manage to find any documentation on how to do this "[OR]". Almost everything I managed to find is related to testing against other key or checking just against an explicit array without the exists "or".

NOTE: This is not case of required_if/unless because I need to check against the currently tested value, not against other key.

via Chebli Mohamed