feat: Change optional Open API query parameters to allow `None

[forest-benchling]

Fixes openapi-generators#285.
Upstream: https://github.com/triaxtec/openapi-python-client/pull/297/files

Instead of methods like

def sync_detailed(
    *,
    client: Client,
    size: Union[Unset, int] = UNSET,
) -> Response[Thing]:

we will now generate

def sync_detailed(
    *,
    client: Client,
    size: Union[Unset, None, int] = None,
) -> Response[Thing]:

and treat None the same as Unset.

This is a bit different from the original proposal in the linked issue, but if you scroll down you can see that it's what was agreed upon in the end IIUC (though I wasn't sure if we agreed on what the default value should be). In addition, it has the benefit of preserving back-compat.

As part of this issue, I also tried to collapse nested Optional and Union types, so that what was formerly Union[Unset, Optional[int]] or Optional[Union[Unset, int]] is now Union[Unset, None, int].