pacat-simple-vchan exits when no output device or microphone is available

[qubesuser]

Qubes OS version:

R4.0-rc2, and I think R3.2 was affected too

Steps to reproduce the behavior:

1. Connect an USB audio device (some sort of dummy PulseAudio sink would likely work as well)
2. Set all other devices to "Off" in pavucontrol's configuration tab (or just not have any other devices)
3. Start a VM, play audio from the VM
4. Unplug the USB audio device
5. Reconnect the USB audio device

Expected behavior:

Audio resumes once the USB audio device is plugged back in.

Also the VM is preferably not aware of the audio device plugging/unplugging, or perhaps it should be configurable whether the VM is told to.

Actual behavior:

pacat-simple-vchan quits upon the USB audio device being unplugged, and thus audio doesn't resume.

General notes:

The problem also happens if there are other devices, but they have no microphone available because they are set to output-only instead of duplex.

Adding a null device by default to PulseAudio works around this, and doing this by default could be a solution; alternatively, pacat could be fixed to work even if it can't create a PulseAudio stream due to no sinks/sources (and of course listen to sink/source changes and recreate the stream when they reappear).