Tethering: check if configManager is null

cherry-picked from b07f1409

Before attempting to retrieve a carrier entitlement setting, first make
sure we have a carrier config manager and if we do have a carrier config
manager, the config it holds is not null.

Bug: 31929638
Test: unittests coming in follow on CL
Change-Id: Iee476c476cb3de109cdccbd955b7fb986dd4267b