Don't fetch calls if the parent doesn't exist.

This CL is a continuation of cl/187700318. In that CL, we were able to unregister
the call log's content observer and prevent this crash, but it's reshowing
itself in voicemail now.

Since the voicemail's observer is registered in the adapter and is unaware of
lifecycle changes, it's much more invasive to fix. So as a quick bandaid, we'll
just check that the parent isn't null.

Bug: 73972084
Test: n/a
PiperOrigin-RevId: 188812159
Change-Id: I533114de66b52395b0f3c90d4ec7a927c4df707f