fix object not found

[FaserF]

This fixes the following two issues, most likely occurring due to WIFI interfaces with no clients.

Traceback (most recent call last):
  File "/config/custom_components/openwrt/coordinator.py", line 300, in async_update_data
    result['wireless'] = await self.update_ap(wireless_config['ap'])
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/coordinator.py", line 227, in update_ap
    result[item['ifname']] = await self.update_hostapd_clients(item['ifname'])
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/coordinator.py", line 131, in update_hostapd_clients
    for key, value in response['clients'].items():
                      ~~~~~~~~^^^^^^^^^^^
KeyError: 'clients'

Traceback (most recent call last):
  File "/config/custom_components/openwrt/coordinator.py", line 300, in async_update_data
    result['wireless'] = await self.update_ap(wireless_config['ap'])
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/coordinator.py", line 227, in update_ap
    result[item['ifname']] = await self.update_hostapd_clients(item['ifname'])
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/coordinator.py", line 125, in update_hostapd_clients
    response = await self._ubus.api_call(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/ubus.py", line 41, in api_call
    return await self._api_call(rpc_method, subsystem, method, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/ubus.py", line 109, in _api_call
    raise NameError(message)
NameError: Object not found

This will most likely fix: #24