Don't update on voicemail fetch if no activity.

The right thing to do is to return early in this case.

This is a theoretical fix for the tagged bug; presuming that this
exception caused code execution to terminate prematurely and delete
the voicemail as it would be expected to.

Bug: 25346075
Change-Id: I643ff4e8abf723f3673b38f5933416730e5d5f9f