WifiScanner: check nullness of bundle extra

In WifiScanner#getAvailableChannels(), check if the
extra is null and return an empty list if that is the
case to ensure that the API upholds its @NonNull
guarantee.

Bug: 143985829
Test: compiles
Change-Id: I5abc2e6362469f77db0fa386adbe06b445aa799c
1 file changed