Simplify Nullness Checks for CC Bundle

Centralize the nullness checks for a  CarrierConfig
Bundle in GsmCdmaPhone to increase safety and remove
some boilerplate. Also, one nullness check was previously
missed, so that is addressed as well.

Bug: 303254005
Test: atest FrameworksTelephonyTests #(non-functional change)
Change-Id: I41deabf97502d839b439e0ec4e06ea760a357806