protonmsg: don't send rpm.sign messages when the sigkey is empty
    
    When a build completes, we store the original, unsigned rpm headers
    to disk. This results in the postRPMSign callback being called, and
    a rpm.sign message being sent with an empty sigkey. The intended
    sematics of the rpm.sign message is to notify a service when an rpm
    has been signed. Therefore, we should not be sending a message when
    the sigkey is empty.