Light: make supported_color_modes None when none are supported

Code returned an empty set, but upstream docs are now updated to clarify
that None is the expected value. This fallback should not be reached for
real devices, but issue #3230 suggests that it is.

custom_components/tuya_local/light.py

@@ -88,7 +88,6 @@ class TuyaLocalLight(TuyaLocalEntity, LightEntity):
                     self.name or "light",
                     self.color_mode,
                 )
-        return set()
 
     @property
     def supported_features(self):