allow no certificates to be reported by the gnutls_certificate_retrieve_function callbacks

In 9829ef9a we introduced a wrapper over the older callback functions which didn't handle this case.

Checklist

• Code modified for feature
• Test suite updated with functionality tests

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code