Check null for getPhoneAccount() in VoicemailChannelUtils

A phone account can be removed after calling getCallCapablePhoneAccounts(), so subsequent getPhoneAccount() can be null.

Bug: 75598828
Test: In a follow up CL, where all instance of these will be fixed.
PiperOrigin-RevId: 189635573
Change-Id: I897db6e8e7706bc8afe6f25d89fd2ca4405900df
1 file changed