[OAS 3.1.0] Operation no longer requires responses

[Azquelt]

It's now valid for an Operation not to have a responses property.

This is primarily to allow people authoring OpenAPI documents to lay out the operations they need before specifying the responses to them, but we may want to:

• check we don't say it's required in our Javadoc
• add a test which creates a document which has an Operation without responses

[Azquelt]

I don't see anywhere that we document that responses is required, so I think we just need a new test. Could test via static file and OASModelReader.

[benjamin-confino]

Please assign this one to me.