Check null for getPhoneAccount()

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

Other usages already has the null check.

Bug: 75598828
Test: N/A testing this involves getPhoneAccount() returning null on the nth call, which requires replacing the shadows with a mock and is highly coupled with the implementation. The is an edge case and is not worth the effort and brittleness it will cause.
PiperOrigin-RevId: 189644833
Change-Id: Ie92dda2537befb5936ad734954b4eaf75964f465