Fix ISY blocking bug (#16978)

This fix results in `is_on` returning False if the state is unknown (which was a bug in 0.79).

homeassistant/components/light/isy994.py

@@ -31,12 +31,14 @@ class ISYLightDevice(ISYDevice, Light):
     @property
     def is_on(self) -> bool:
         """Get whether the ISY994 light is on."""
+        if self.is_unknown():
+            return False
         return self.value != 0
 
     @property
     def brightness(self) -> float:
         """Get the brightness of the ISY994 light."""
-        return self.value
+        return None if self.is_unknown() else self.value
 
     def turn_off(self, **kwargs) -> None:
         """Send the turn off command to the ISY994 light device."""