Fix bug when there is no previous state

Before it would get errors like this:

[2014/01/13 21:17:52 UTC] pid:1255 ERROR nymms.reactor.handlers.Handler Handler(_filter):43 - Filter not_soft_recovery on Handler SESHandler had an unhandled exception. Ignoring:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nymms/reactor/handlers/Handler.py", line 39, in _filter
    results.append(f(result, previous_state))
  File "/usr/lib/python2.7/dist-packages/nymms/reactor/filters/__init__.py", line 81, in not_soft_recovery
    if previous_state.state_type == results.SOFT:
AttributeError: 'NoneType' object has no attribute 'state_type'