Support nullable as type arrays for OpenAPI 3.1

[timomeh]

In OpenAPI v3.1 nullable: true was removed in favor of type arrays with the value "null". Reference: https://www.openapis.org/blog/2021/02/16/migrating-from-openapi-3-0-to-3-1-0

# OpenAPI v3.0
type: string
nullable: true

# OpenAPI v3.1
type:
- "string"
- "null" 

The current version of openapi-typescript generates those properties as unknown, instead of string | null.

[drwpow]

I think there are 2 things here worth addressing:

1. This feels like a good change that should happen for both 3.0 and 3.1

I think the unknown behavior is probably improved by a string | null union as you suggested. I think I remember some previous changes to the nullable flag, but I don’t recall an argument against having string | null generated (but if I’m wrong or if someone finds a previous decision in the issues this ticket disagrees with, please remind me!).

2. This library should probably distinguish between 3.0 and 3.1

This doesn’t block the previous solution from happening, but it may be needed in the future.

Extra info

There is an internal version number that gets passed around for the parts of the codegen where Swagger 2.x differs from OpenAPI 3.x. But where OpenAPI 3.0 differs from 3.1, this library could do better.

I think this wouldn’t be a major change—probably just switching that internal version number to a discriminated union (e.g. 2 | 3 | 3.1 or perhaps a string equivalent). That should probably happen at some point.

Sidenote: previous versions of this library did have more of a separation between 2.x and 3.x schemas, but all-in-all they share so much in common that it was easier to maintain one codebase and hard-code in the deviations rather than maintain 2 completely-isolated generators

Anyways, would love if someone could open a PR to address this!

[BrandonGoren]

@drwpow I have a PR open that I think solves this issue by handling array types recursively: #917