SMPP raises error when trying to parse Safaricom delivery reports.

This is the verbose log:

```
2012-06-14 11:02:32+0000 [EsmeReceiver,client] INCOMING <<<< 000000db00000005000000000000000232323736320001013235343731363233373636330000003232373632000400000000000000007a69643a31363037303632303032207375623a30303120646c7672643a303031207375626d697420646174653a3132303631343134303220646f6e6520646174653a3132303631343134303220737461743a44454c49565244206572723a30303020746578743a54657374696e6720323237363220612073657665001e000b3136303730363230303200042700010206060001001501000d32353437323235303036313200
2012-06-14 11:02:32+0000 [EsmeReceiver,client] INCOMING <<<< {'body': {'mandatory_parameters': {'priority_flag': 0, 'source_addr': '254716237XXX', 'protocol_id': 0, 'replace_if_present_flag': 0, 'registered_delivery': 0, 'dest_addr_ton': 'unknown', 'source_addr_npi': 'ISDN', 'schedule_delivery_time': '', 'dest_addr_npi': 'unknown', 'sm_length': 122, 'esm_class': 4, 'data_coding': 0, 'service_type': '22762', 'source_addr_ton': 'international', 'sm_default_msg_id': 0, 'validity_period': '', 'destination_addr': '22762', 'short_message': 'id:1607062002 sub:001 dlvrd:001 submit date:1206141402 done date:1206141402 stat:DELIVRD err:000 text:Testing 22762 a seve'}, 'optional_parameters': [{'length': 11, 'tag': 'receipted_message_id', 'value': '1607062002'}, {'length': 1, 'tag': 'message_state', 'value': 2}, {'length': 1, 'tag': 'broadcast_area_identifier', 'value': '00'}, {'length': 13, 'tag': '1501', 'value': '32353437323235303036313200'}]}, 'header': {'command_status': 'ESME_ROK', 'command_length': 219, 'sequence_number': 2, 'command_id': 'deliver_sm'}}
2012-06-14 11:02:32+0000 [EsmeReceiver,client] OUTGOING >>>> {'body': {'mandatory_parameters': {'message_id': ''}}, 'header': {'command_status': 'ESME_ROK', 'command_length': 17, 'sequence_number': 2, 'command_id': 'deliver_sm_resp'}}
2012-06-14 11:02:32+0000 [EsmeReceiver,client] PUBLISHING DELIV REPORT: None delivered
2012-06-14 11:02:32+0000 [EsmeReceiver,client] Unhandled error in Deferred:
2012-06-14 11:02:32+0000 [EsmeReceiver,client] Unhandled Error
    Traceback (most recent call last):
      File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 1045, in _inlineCallbacks
        result = g.send(result)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/middleware/message_storing.py", line 68, in handle_event
        yield self.store.add_event(event)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/model.py", line 231, in wrapper
        return manager.call_decorator(func)(self, *args, **kw)
      File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 1187, in unwindGenerator
        return _inlineCallbacks(None, gen, Deferred())
    --- <exception caught here> ---
      File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 1045, in _inlineCallbacks
        result = g.send(result)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/message_store.py", line 150, in add_event
        event_record = self.events(event_id, event=event, message=msg_id)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/model.py", line 319, in __call__
        return self._modelcls(self._manager, key, **data)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/model.py", line 101, in __init__
        descriptor.initialize(self, field_value)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/fields.py", line 579, in initialize
        self.set_foreign_object(modelobj, value)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/fields.py", line 586, in set_foreign_object
        self.validate(otherobj)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/fields.py", line 33, in validate
        self.field.validate(value)
      File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/fields.py", line 103, in validate
        raise ValidationError("None is not allowed as a value for non-null"
    vumi.persist.fields.ValidationError: None is not allowed as a value for non-null fields.
    
2012-06-14 11:02:32+0000 [EsmeReceiver,client] STATE: BOUND_RX
```