Handle case when last_policy_change is None
Handle case when last_policy_change is None

The previous version assumed that last_policy_change is always non-None.
However, the timing can work out such that nothing calls
receive_policy_update, before the first retry, and therefore hte
last_policy_change is None.  This causes intermittent test failurse.

Change-Id: I3e1200331303062caba1e10229482a0ef41807af

ramineni/my_congress,ekcs/congress,ekcs/congress,ramineni/my_congress,openstack/congress,openstack/congress,ramineni/my_congress,ramineni/my_congress,ekcs/congress,ekcs/congress