level is returned as None in hidpp10.py when the performanceMX mouse is

charging. Since the battery state is unknown when beeing recharged it
will now return "N/A".
Before it would crash becasue level was None and could not be inserted
using %d.

lib/solaar/cli/show.py

@@ -107,10 +107,13 @@ def _print_device(dev):
 		if battery is not None:
 			from logitech_receiver.common import NamedInt as _NamedInt
 			level, status = battery
-			if isinstance(level, _NamedInt):
-				text = str(level)
+			if level:
+				if isinstance(level, _NamedInt):
+					text = str(level)
+				else:
+					text = '%d%%' % level
 			else:
-				text = '%d%%' % level
+				text = 'N/A'
 			print ('     Battery: %s, %s.' % (text, status))
 		else:
 			print ('     Battery status unavailable.')