Fix wake_on_lan ping with None as host (#6532)

* Update configuration validation

With the new update, wake_on_lan requires a host key in the configuration

* cast self._host to str as requested

* Changed host key back to optional

homeassistant/components/switch/wake_on_lan.py

@@ -86,10 +86,10 @@ class WOLSwitch(SwitchDevice):
         """Check if device is on and update the state."""
         if platform.system().lower() == 'windows':
             ping_cmd = ['ping', '-n', '1', '-w',
-                        str(DEFAULT_PING_TIMEOUT * 1000), self._host]
+                        str(DEFAULT_PING_TIMEOUT * 1000), str(self._host)]
         else:
             ping_cmd = ['ping', '-c', '1', '-W',
-                        str(DEFAULT_PING_TIMEOUT), self._host]
+                        str(DEFAULT_PING_TIMEOUT), str(self._host)]
 
         status = sp.call(ping_cmd, stdout=sp.DEVNULL)
         self._state = not bool(status)