default unit to None in sensor decorator (#1590)

Split out from: #1586
fix sensor unit beeing set to "" instead of None (HomeAssistant default)

Author: starkillerOG

miio/devicestatus.py

@@ -131,7 +131,7 @@ def __getattr__(self, item):
         return getattr(self._embedded[embed], prop)
 
 
-def sensor(name: str, *, unit: str = "", **kwargs):
+def sensor(name: str, *, unit: Optional[str] = None, **kwargs):
     """Syntactic sugar to create SensorDescriptor objects.
 
     The information can be used by users of the library to programmatically find out what