volume_status module: pamixer: Use default device if none specified

Not specifying the device should use whatever pamixer thinks is the default.
Using 0 might not be always the default, especially because the default (or
whatever pamixer thinks is the default) can change at runtime.

py3status/modules/volume_status.py

@@ -172,7 +172,7 @@ def toggle_mute(self):
 class Pamixer(Audio):
     def setup(self, parent):
         is_input = "--source" if self.is_input else "--sink"
-        self.cmd = ["pamixer", "--allow-boost", is_input, self.device or "0"]
+        self.cmd = ["pamixer", "--allow-boost"] + ([is_input, self.device] if self.device else [])
 
     def get_volume(self):
         try: