don't crash when the server doesn't respond

inside libpulse a non-reply (e.g. caused by a timeout) results in info
being a nullptr. when that happens simply skip over the callback. when
this happens chances are the server crashed or is otherwise defunct so
we won't be able to do much about this anyway

easy to test by attaching to both plasmashell and pulseaudio and
interrupting the latter when the former calls
pa_context_get_server_info. this results in the reply timeout getting
hit -> nullptr callback.

it is unclear if we can somehow recover from this but in lieu of a
reliable real world test case for this we at least shouldn't crash on
nullptr access.

BUG: 454647
BUG: 437272


(cherry picked from commit e87f2b13)