The unit of sensor.charger_energy_reactive_import_register has changed

[mathoudebine]

Describe the bug
After a restart of HomeAssistant server, the following message is displayed in Settings > System > Repairs:

The unit of sensor.charger_energy_reactive_import_register has changed

The unit of 'Energy Reactive Import Register' (sensor.charger_energy_reactive_import_register) changed to 'kWh' which can't be converted to the previously stored unit, 'varh'.
If the historic statistic values have a wrong unit, you can update the units of the old values. The values will not be updated.
Otherwise you can choose to delete all historic statistic values, and start over.

To Reproduce
Steps to reproduce the behavior:

1. Add a OCPP 1.6 chargepoint that offers Energy Reactive Import Register values (I suspect it may be the same with Energy Reactive Export Register or Energy Reactive Import/Export Interval)
2. Restart HomeAssistant or temporarily lose connection to the charger e.g. network disconnection
3. The repair message appears and the sensor unit is incorrect:
   

Expected behavior
Units for Energy Reactive ... sensors do not change and stay in varh like indicated in the OCPP 1.6 documentation:

Additional context
It may be caused by this code but not sure:

ocpp/custom_components/ocpp/sensor.py

Lines 193 to 202 in f4ed62e

	def native_unit_of_measurement(self):
	"""Return the native unit of measurement."""
	value = self.central_system.get_ha_unit(self.cp_id, self.metric)
	if value is not None:
	self._attr_native_unit_of_measurement = value
	else:
	self._attr_native_unit_of_measurement = DEFAULT_CLASS_UNITS_HA.get(
	self.device_class
	)
	return self._attr_native_unit_of_measurement