When unregistering callback due to ON_UNAVAILABLE did not check for a non-null callback. Bug: 132950880 Test: atest ConnectivityServiceTest Merged-In: Ib3fde31d88c36469cdee1e3578606d130a9817cb Change-Id: Ib3fde31d88c36469cdee1e3578606d130a9817cb (cherry picked from commit 51ddc176abd23bd3ddbc26124e5541a983a1db07)