[#350] password input to AUTH_RESPONSE_AN should be string

The default password argument value for _login_native( ) is None,
which (for the password-less anonymous login case) caused problems
in the string formatting for the AuthResponsemessage to the server.
An empty string turns out to be more appropriate.

Author: d-w-moore

irods/connection.py

@@ -493,7 +493,7 @@ def _login_native(self, password=None):
 
         # Default case, PAM login will send a new password
         if password is None:
-            password = self.account.password
+            password = self.account.password or ''
 
         # authenticate
         auth_req = iRODSMessage(msg_type='RODS_API_REQ', int_info=703)