ha-asusrouter: [Bug] RT-AC87U in Media Bridge mode with v0.17.1 - object of type 'NoneType' has no len()

The problem

RT-AC87U in Media Bridge mode throws an error when loading after updating from v0.16.1 to v0.17.1

TypeError: object of type 'NoneType' has no len()

Your device model

RT-AC87U

Firmware type

Merlin

Firmware version

3.0.0.4.384.13_10

Integration version with the issue

0.17.1

Method of the integration installation

HACS

What version and type of Home Assistant installation do you use

2023.1.2

Is there anything useful in the logs?

Logger: homeassistant.config_entries
Source: helpers/device_registry.py:129
First occurred: 09:51:06 (1 occurrences)
Last logged: 09:51:06

Error setting up entry 192.168.45.240 for asusrouter
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/config_entries.py", line 382, in async_setup
    result = await component.async_setup_entry(hass, self)
  File "/config/custom_components/asusrouter/__init__.py", line 24, in async_setup_entry
    await router.setup()
  File "/config/custom_components/asusrouter/router.py", line 665, in setup
    entry.entity_id, new_unique_id=f"{self.mac}_{uid}"
  File "/config/custom_components/asusrouter/router.py", line 1097, in mac
    return format_mac(self._identity.mac)
  File "/usr/src/homeassistant/homeassistant/helpers/device_registry.py", line 129, in format_mac
    if len(to_test) == 17 and to_test.count(":") == 5:
TypeError: object of type 'NoneType' has no len()

Diagnostics information

No response

Additional information

No response