OpenAPI 3.1.0 support: handle cases of missing `baseDoc` URI

[char0n]

The goal of this issue is to handle cases when baseDoc is not provided as part of resolver options. This can happen if spec object is provided directly to the resolver.

First we need to figure out how OpenAPI 2.0 and OpenAPI 3.0.x resolution strategy behaves when baseDoc is not provided and then this behavior needs to replicated to OpenAPI 3.1.0 resolution strategy.