Do not crash on last_clean_details when no history available

[rytilahti]

From #456

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/homeassistant/helpers/entity_platform.py", line 248, in _async_add_entity
    await entity.async_device_update(warning=False)
  File "/usr/local/lib/python3.6/site-packages/homeassistant/helpers/entity.py", line 349, in async_device_update
    await self.hass.async_add_executor_job(self.update)
  File "/usr/local/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/local/lib/python3.6/site-packages/homeassistant/components/vacuum/xiaomi_miio.py", line 378, in update
    self.last_clean = self._vacuum.last_clean_details()
  File "/usr/local/lib/python3.6/site-packages/miio/vacuum.py", line 194, in last_clean_details
    last_clean_id = self.clean_history().ids.pop(0)
IndexError: pop from empty list