[CM] Fix NPE due to unvalidated callback value

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)
2 files changed