Fetching OCSP responses may result in TypeError

First of all, thank you for certvalidator! It makes an important task easy to complete in Python. 👍

While I was experimenting with certificate revocation status checking I ran into some issues. I'll create separate GitHub issues for those.

When certvalidator tries to fetch OCSP responses, but the certificate to be validated does not contain any reference to OCSP responders, TypeError is raised:

TypeError: exceptions must derive from BaseException

The root of that error is that cert.ocsp_urls returns an empty list, so at the end of the following code block None is raised:

https://github.com/wbond/certvalidator/blob/5bc5c390c1955195507c23db91b8926bb03f7385/certvalidator/ocsp_client.py#L89-L109

The cert.ocsp_urls == [] case could be handled here (for safety) e.g. by raising an appropriate error, but in that case ocsp_client.fetch probably shouldn't be called at all, since it can not return any meaningful result.

Start in certvalidator/ocsp_client.py around lines 89-109 and inspect how an empty cert.ocsp_urls list reaches the fetch logic. Reproduce validation with a certificate that has no OCSP responder reference, then define handling that avoids raising None as an exception and confirms ocsp_client.fetch is not called when no responder is available.