OpenAPI 3.0 and nullable not tested

[Relequestual]

I was looking at... something else, and noticed OAD 3.0 with nullable defined.
The OAD is used in a project that uses openapi-backend.
I wonderd how validation was working.
I noticed it uses ajv.

I know ajv isn't capable of correctly validating OpenAPI 3.0. It is not OpenAPI aware.

One specific issue is nullable differs from OAS 3.0 to 3.1.

Looking at the tests, nullability is being tested for 3.1, but not 3.0.

It looks like validation for OpenAPI 3.0 isn't tested at all.