Remove state class "measurement" when there is no UOM

This should fix the newly introduced strict check that causes this sensor to fail

custom_components/purpleair/sensor.py

@@ -81,7 +81,7 @@ def native_value(self):
 
     @property
     def state_class(self):
-        return 'measurement'
+        return 'measurement' if self._uom is not None else None
 
     @property
     def unique_id(self):