tls1.3: server returns early on handshake when no cert is provided by client

This modifies TLS 1.3 handshake on server side to return early when possible.

Checklist

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

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