[SB] Fix null carrier config during dump.

It's theoretically possible for a carrier config to be removed while
`CarrierConfigRepositoryImpl` is being dumped, so protect against that
case by checking if the config is null before dumping it.

Fixes: 386884215
Flag: EXEMPT bugfix
Test: atest CarrierConfigRepositoryImplTest
Change-Id: I3285e0c10e5ac806dc08d587415ba08294bdf071