Error when we delete the messages from an empty inbox

[Leo Arias]

When we try to delete the messages from an inbox without message, the following error occurs:

  File "/mnt/jenkins/workspace/canonical-identity-provider-acceptance-dev-u1/working/.env/local/lib/python2.7/site-packages/u1testutils/mail.py", line 123, in delete_msgs_sent_to
    mbox.delete_msgs_sent_to(email_address)
  File "/mnt/jenkins/workspace/canonical-identity-provider-acceptance-dev-u1/working/.env/local/lib/python2.7/site-packages/u1testutils/mail.py", line 108, in delete_msgs_sent_to
    for num in reversed(data[0].split()):
AttributeError: 'NoneType' object has no attribute 'split'

We should first check if there are messages to delete.