rfc3164: ommit [pid] part if no pid is available

This leaves the [pid] part off if no valid pid is given when using the rfc3164 output. 

The rfc is vague about what is allowed in the pid part, I check the pid to be an integer as I've never seen non-integers in the pid part in the wild and I have seen applications break when encountering non-integer values between the brackets.