Use None as initial state in zha component (#14389)

* Return None if state is unknown

* Use None as initial state

homeassistant/components/binary_sensor/zha.py

@@ -108,7 +108,7 @@ class BinarySensor(zha.Entity, BinarySensorDevice):
     @property
     def is_on(self) -> bool:
         """Return True if entity is on."""
-        if self._state == 'unknown':
+        if self._state is None:
             return False
         return bool(self._state)