When gvc gives us an active-input/output-device ID we don't know or sets the ID
to 0, that means there's no device currently active. While this seems unlikely
to be the case without the device getting removed at the same time, it can happen:
When the bluetooth profile gets changed from handset (input+output) to
headphone (output only), the input device remains available, but there's no
more active-input-device anymore.
In this case, we shouldn't leave the active entry on the device combo box as-is,
instead we should set it to NULL to indicate that no input device is set.
This fixes a bug where the input switcher is not updated when there's no
internal microphone but there's a bluetooth headset connected, and the
bluetooth profile gets switched from Handset to Headphone.
afec106a8b
