Fix string formatting when port is not given.
Fix string formatting when port is not given.

If port is None, we would try to format an empty string with %d wich
fails. Fix it by using %s.

Reported-by: Iain Dalton <28e8a4191993b525e2e5e8727283a3562a8153db@gmail.com>
Signed-off-by: Sebastian Spaeth <98dcb2717ddae152d5b359c6ea97e4fe34a29d4c@SSpaeth.de>

frioux/offlineimap,frioux/offlineimap