Fix iterating over NoneType exception (#23648)

* Fix iterating over NoneType exception

When self._dark_sky is None, don't try to return self._dark_sky.units

* Fix wrong check

homeassistant/components/darksky/weather.py

@@ -103,6 +103,8 @@ class DarkSkyWeather(WeatherEntity):
     @property
     def temperature_unit(self):
         """Return the unit of measurement."""
+        if self._dark_sky.units is None:
+            return None
         return TEMP_FAHRENHEIT if 'us' in self._dark_sky.units \
             else TEMP_CELSIUS