input: Write a python function to filter inverters by their 'device_type' from a given list of 'FroniusSim' objects and return their 'custom_name'.
  assert filter_inverters_by_device_type(inverters, 232) == ['SouthWest']
  assert filter_inverters_by_device_type(inverters, 224) == ['']
  assert filter_inverters_by_device_type(inverters, 208) == ['Tmp']
  assert filter_inverters_by_device_type(inverters, None) == ['SMA']
output: def filter_inverters_by_device_type(inverters, device_type):
      return [inverter['custom_name'] for inverter in inverters if inverter['device_type'] == device_type]