Only strip from the bluetooth name if it isn't None

[d0ugal]

This prevents the following traceback that will otherwise occur.

Traceback (most recent call last):
  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/device_tracker/bluetooth_le_tracker.py", line 107, in update_ble
    see_device(address, devs[address], new_device=True)
  File "/usr/local/lib/python3.6/site-packages/homeassistant/components/device_tracker/bluetooth_le_tracker.py", line 47, in see_device
    see(mac=BLE_PREFIX + address, host_name=name.strip("\x00"),
AttributeError: 'NoneType' object has no attribute 'strip'

Description:

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

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• Documentation added/updated in home-assistant.io

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• Tests have been added to verify that the new code works. NOTE: I don't see any current test coverage for this component. Have I just missed the tests?