Allow omitting Presence

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.

Start by locating how Presence is applied to non-required fields in the OpenAPI code-generation paths. Define the option's intended behavior and verify that generated fields can omit Presence while treating non-required fields as nullable; add or update coverage for the affected generation cases.

Written by the indexing model from the issue text.