Fix the case that "/sys/class/power_supply" is not existing

This fixes the execution on FreeBSD.

bpytop.py

@@ -1637,12 +1637,12 @@ class CpuBox(Box, SubBox):
 			return False
 
 		if cls.battery_path == "":
+			cls.battery_path = None
+			if os.path.isdir("/sys/class/power_supply"):
 				for directory in os.listdir("/sys/class/power_supply"):
 					if directory.startswith('BAT') or 'battery' in directory.lower():
 						cls.battery_path = f'/sys/class/power_supply/{directory}/'
 						break
-				else:
-					cls.battery_path = None
 
 		return_true: bool = False
 		percent: int = ceil(getattr(psutil.sensors_battery(), "percent", 0))