Correct auth without password or username.

If no username is set, returns an empty string. So the test `username is
not None` is always True. Idem for password. This can lead to
authentication problems. This commit fixes the test to `if username and
password` as it was in the previous version.

isso/ext/notifications.py

@@ -72,7 +72,7 @@ class SMTP(object):
 
         username = self.conf.get('username')
         password = self.conf.get('password')
-        if username is not None and password is not None:
+        if username and password:
             if PY2K:
                 username = username.encode('ascii')
                 password = password.encode('ascii')