allow empty certificate/key

[trefzer]

if client=true, the certificate and the private key is not needed.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[trefzer]

Sorry, but requesting a CLA to sign, for a patch that is pending for 4 years is a little bit to much !
For signing you want to access my github profile (ok only for email, which you already know from commit) which I will not agree !

So, either you take it (and I hereby guarantee, that I wrote the if statement myself and everybody can do whatever he wants with it), or you can close the request (but then without permission to add the functionality this way !)