SMPP transport sometimes gives invalid user_message_id in events.

The error looks like:
```
u'Unable to find message for event: <Message payload="{\'transport_name\': u\'netcore_india_smpp_transport\', \'event_type\': u\'delivery_report\', \'event_id\': u\'REDACTED\', \'timestamp\': datetime.datetime(2014, 4, 13, 10, 15, 30, 225255), \'routing_metadata\': {}, \'message_version\': u\'20110921\', \'helper_metadata\': {\'go\': {}}, \'delivery_status\': u\'delivered\', \'transport_metadata\': {}, \'user_message_id\': u\'None\', \'message_type\': u\'event\'}">'
```
Note the ``'user_message_id': u'None'``

Configuration which produced this has:
```yaml
deliver_short_message_processor_config:
  # Because they lie about their unicode encoding
  data_coding_overrides:
    0: 'utf-8'
    8: 'utf-8'

submit_short_message_processor_config:
  # allow for multipart SMS delivery
  send_multipart_udh: True
```