[OAS 3.1.0] Only one of Paths, Components, or Webhooks is required

[Azquelt]

Previously paths was always required in an OpenAPI document, but now only one of paths, components and webhooks is required.

In practise, I suspect that usually we will almost always have paths when using MP OpenAPI.

However, we may want to:

• Check and update our documentation in this area
• Add a test building a model without a paths entry

[Azquelt]

I can't see anywhere we're requiring paths, so we just need a test where we build a model without paths. This would need to be a separate app and it should probably disable scanning and just build the model with OASModelReader.