Enforce null check for mAccessPoint

mAccessPoint == null implies this is a new network.
In this case, we don't need to check if it's a carrier AP.

Bug: 68846574
Test: compile, manually test that crash does not happen again.
Test: make RunSettingsRoboTests -j40 aROBOTEST_FILTER=WifiConfigControllerTest

Change-Id: I03b1da5e8f0b8e4270c0c52c67bbc90c999be078
(cherry picked from commit 0117b995)