Use 'not' for handling empty messages

We've been getting this type of error in the past few days:

File "build/bdist.linux-x86_64/egg/iron_celery/iron_mq_transport.py", line 41, in _get
    if len(messages["messages"]) == 0:
TypeError: object of type 'NoneType' has no len()

This pull request fixes it.