Features conformance class validation includes requests to /collections/None

[mattskone]

stac-api-validator version 0.6.1
Python version 3.10.12
Mac OS Ventura 13.4

Steps to reproduce:

1. Run stac-api-validator http://localhost:8080/api/v1/stac/ --conformance features

Actual behavior: validation errors include

- [Collections] : GET http://localhost:8080/api/v1/stac/collections/None params=None body= had unexpected status code 404 instead of 200:
- [Collections] : http://localhost:8080/api/v1/stac/collections/None body was empty

Expected behavior:

1. If --collection is a required option when validating the features conformance class, this requirement is documented in the usage docs, validated at the CLI entrypoint, and the application terminates if the required option is not used.
2. If not, the collections/{collection_id} endpoint is not tested when --collection is not used (or validate_collections() responds gracefully to the absence of a collection_id).

[mattskone]

I do note that the required use of --collection when validating the features conformance class is documented here.