trusty-mitaka threw a hook error on cluster joined

[Chris MacNaughton]

Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/charms/reactive/__init__.py", line 72, in main
    bus.dispatch(restricted=restricted_mode)
  File "/usr/local/lib/python3.4/dist-packages/charms/reactive/bus.py", line 375, in dispatch
    _invoke(other_handlers)
  File "/usr/local/lib/python3.4/dist-packages/charms/reactive/bus.py", line 351, in _invoke
    handler.invoke()
  File "/usr/local/lib/python3.4/dist-packages/charms/reactive/bus.py", line 173, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-designate-bind-0/charm/reactive/designate_bind_handlers.py", line 74, in update_zones_from_peer
    designate_bind.retrieve_zones(hacluster)
  File "lib/charm/openstack/designate_bind.py", line 112, in retrieve_zones
    DesignateBindCharm.singleton.retrieve_zones(cluster_relation)
  File "lib/charm/openstack/designate_bind.py", line 440, in retrieve_zones
    if request_time and request_time > sync_time:
TypeError: unorderable types: str() > NoneType()