Fix power-on key None guard in fan base class (#553)

When neither POWERON_KEY nor FANON_KEY is in the initial state,
_power_on_key was set to None. This caused _send_command(None, value)
in the is_on setter. Now preserves existing key or defaults to
POWERON_KEY, and adds a guard in the setter.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: JeffSteinbok

custom_components/dreo/pydreo/pydreofanbase.py

@@ -127,6 +127,9 @@ def is_on(self):
     def is_on(self, value: bool):
         """Set if the fan is on or off"""
         _LOGGER.debug("is_on: is_on.setter - %s", value)
+        if self._power_on_key is None:
+            _LOGGER.error("is_on: Cannot set power state — power on key is unknown")
+            return
         self._send_command(self._power_on_key, value)
 
     @property
@@ -346,7 +349,9 @@ def update_state(self, state: dict):
                 self._power_on_key = FANON_KEY
             else:
                 _LOGGER.error("update_state: Unable to get power on state from state. Check debug logs for more information.")
-                self._power_on_key = None
+                # Default to POWERON_KEY so is_on setter doesn't send None key
+                if self._power_on_key is None:
+                    self._power_on_key = POWERON_KEY
 
         self._fan_speed = self.get_state_update_value(state, WINDLEVEL_KEY)
         if self._fan_speed is None: