Give an option to mark not required fields as nullable

[gyz0072004]

Checklist

• [x] Conversion: I have checked my source definition is valid OpenAPI 2.0

Detailed Description

Our API would return NULL value for optional fields in swagger definition. It's acceptable in OpenAPI 2.0. However, because the transformed OpenAPI 3.0 definition doesn't mark the field as nullable, the field becomes not nullable. We would like the toolkit to support auto marking the fields as nullable

Other stuff

swagger2openapi: 7.0.8

[MikeRalphson]

Could you expand on why this behaviour was acceptable in OpenAPI 2.0 (it doesn't seem obvious to me). Optional and nullable mean different things here. My gut feeling is this isn't something I'd be likely to add, but please try and convince me!