Allow for None response from wlanGetCountry

If an interface is down when the tests query for a country code,
the response will be None. This shouldn't happen often, but is
possible and not necessarily indicative of an error. Added logic
to handle None response when configure country code.

Bug: None
Test: Ran regulatory tests to verify.
Change-Id: I6d0c72f092a3f494d7d987c3c3f52cfb409ccf3b