allow kojid to start when not using ssl cert auth
    
    kojid in koji 1.12.0 fails to start when not using ssl cert auth
    Traceback (most recent call last):
       File "/usr/sbin/kojid", line 5627, in <module>
         if os.path.isfile(options.cert):
       File "/usr/lib64/python2.7/genericpath.py", line 37, in isfile
         st = os.stat(path)
     TypeError: coercing to Unicode: need string or buffer, NoneType found
    if you do not have a cert option defined we should move on. so add
    to the if statement a check that the option is actually defined
    
    Signed-off-by: Dennis Gilmore <dennis@ausil.us>