Improve data["select_dhw_mode"]

Author: bouwew

plugwise/helper.py

@@ -411,7 +411,7 @@ def _appliance_measurements(
                     case "elga_status_code":
                         data["elga_status_code"] = int(appl_p_loc.text)
                     case "select_dhw_mode":
-                        data["select_dhw_mode"] = appl_p_loc.text
+                        data["select_dhw_mode"] = appl_p_loc.text or None
 
                 common_match_cases(measurement, attrs, appl_p_loc, data)