[bug] openwisp-get-address not work with IPv6 only interface

[Bartosz-lab]

Describe the bug
When the management_interface is configured with only an IPv6 address, openwisp-get-address fails to return any address.

Steps To Reproduce
Steps to reproduce the behavior:

1. Configure management_interface in the settings.
2. Assign only an IPv6 address to this interface.
3. Run the command: /etc/init.d/openwisp_config restart
4. Observe the error in the logs:

   Sat Oct 19 19:00:37 2024 daemon.info openwisp: OpenWISP config agent started
   Sat Oct 19 19:00:37 2024 daemon.notice openwisp: management interface not ready (attempt 1 of 3)
   Sat Oct 19 19:00:47 2024 daemon.notice openwisp: management interface not ready (attempt 2 of 3)
   Sat Oct 19 19:00:57 2024 daemon.notice openwisp: management interface not ready (attempt 3 of 3)
   Sat Oct 19 19:00:57 2024 daemon.warn openwisp: could not determine ip address of management interface, giving up...
   

Expected behavior
The openwisp-get-address script should correctly return the assigned IPv6 address.