Cast fix

[balloob]

Description:
On startup, it can be for the cast devices that the media status has not been received yet.

Related issue (if applicable):

16-12-15 19:54:53 homeassistant.core: Error doing job: Task exception was never retrieved
Traceback (most recent call last):
  File "uvloop/future.pyx", line 372, in uvloop.loop.BaseTask._fast_step (uvloop/loop.c:105885)
  File "/usr/src/app/homeassistant/helpers/entity_component.py", line 335, in _async_process_entity
    new_entity, self, update_before_add=update_before_add
  File "/usr/src/app/homeassistant/helpers/entity_component.py", line 204, in async_add_entity
    yield from entity.async_update_ha_state()
  File "/usr/src/app/homeassistant/helpers/entity.py", line 224, in async_update_ha_state
    attr = self.state_attributes or {}
  File "/usr/src/app/homeassistant/components/media_player/__init__.py", line 699, in state_attributes
    in ATTR_TO_PROPERTY if getattr(self, attr) is not None
  File "/usr/src/app/homeassistant/components/media_player/__init__.py", line 699, in <dictcomp>
    in ATTR_TO_PROPERTY if getattr(self, attr) is not None
  File "/usr/src/app/homeassistant/components/media_player/cast.py", line 248, in media_position
    self.media_status_received).total_seconds()
TypeError: unsupported operand type(s) for -: 'datetime.datetime' and 'NoneType'