Allow omitting Presence

[OlegYch]

Some (if not most) specs don't actually distinguish between a field being non-required and being null.
So it would be nice to avoid using Presence for them.

Ie, please allow an option to treat non-required fields as nullable.

[blast-hardcheese]

@OlegYch Can you share whether encodeOptionalAs=codingOptionalLegacy, decodeOptionalAs=codingOptionalLegacy is the behavior you're looking for?

[OlegYch]

@blast-hardcheese that breaks something in api behavior, but doesn't seem to change generated api code, ie Presence is still everywhere