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

Same as plasma/plasma-pa!142 (merged) -- please discuss there