Fix Tuya Light without brightness (#26534)

Return none if there is brightness data from tuya

https://github.com/home-assistant/home-assistant/issues/25896

homeassistant/components/tuya/light.py

@@ -40,6 +40,8 @@ class TuyaLight(TuyaDevice, Light):
     @property
     def brightness(self):
         """Return the brightness of the light."""
+        if self.tuya.brightness() is None:
+            return None
         return int(self.tuya.brightness())
 
     @property