A supply nobody could read is unknown, not empty

plugins/printers/plugin.py

@@ -323,7 +323,9 @@ class PrintersPlugin(BasePlugin):
 
                 click.echo(f'Supply levels for {ip}:')
                 for supply in supplies:
-                    click.echo(f"  {supply['name']}: {supply['level']}%")
+                    level = supply.get('level')
+                    shown = 'unknown' if level is None else f'{level}%'
+                    click.echo(f"  {supply['name']}: {shown}")
 
         @printerscli.command('seed-supplies')
         def seedsuppliescommand():