Avoid key_error in modbus climate with non-defined fan_mode. (#110017)

homeassistant/components/modbus/climate.py

@@ -364,7 +364,9 @@ async def async_update(self, now: datetime | None = None) -> None:
 
             # Translate the value received
             if fan_mode is not None:
-                self._attr_fan_mode = self._fan_mode_mapping_from_modbus[int(fan_mode)]
+                self._attr_fan_mode = self._fan_mode_mapping_from_modbus.get(
+                    int(fan_mode), self._attr_fan_mode
+                )
 
         # Read the on/off register if defined. If the value in this
         # register is "OFF", it will take precedence over the value