Fix phone process crash due to null carrier config

Perform null pointer check on the carrier config bundle subset return
from CarrierConfigManager#getConfigForSub call to make sure it will
not crash the phone process.

Bug: 266962389
Test: atest FrameworksTelephonyTests
Change-Id: I2f142279d267705fa678e8b632228eef475d7f6f
Merged-In: I2f142279d267705fa678e8b632228eef475d7f6f
(cherry picked from commit bc8832aa)