Fix small bug when no key given to client

IPython/zmq/parallel/client.py

@@ -231,7 +231,7 b' class Client(object):'
                 password = getpass("SSH Password for %s: "%sshserver)
         ssh_kwargs = dict(keyfile=sshkey, password=password, paramiko=paramiko)
         
-        if os.path.isfile(exec_key):
+        if exec_key is not None and os.path.isfile(exec_key):
             arg = 'keyfile'
         else:
             arg = 'key'