Improve hourly_reset_time property

Author: bouwew

plugwise_usb/nodes/helpers/pulses.py

@@ -107,12 +107,13 @@ def collected_logs(self) -> int:
         return counter
 
     @property
-    def hourly_reset_time(self) -> datetime:
+    def hourly_reset_time(self) -> datetime | None:
         """Provide the device hourly pulse reset time.""" 
         if timestamp := self.cons_last_hourly_reset is not None:
             return timestamp
         if timestamp := self.prod_last_hourly_reset is not None:
             return timestamp
+        return None
 
     @property
     def logs(self) -> dict[int, dict[int, PulseLogRecord]]: