We no longer validate that there's an IP address defined.
Seems like Coturn can start without one as well, so there's no need to
require it.

If people populate `matrix_coturn_turn_external_ip_addresses` directly
to specify multiple addresses, they can leave
`matrix_coturn_turn_external_ip_address` empty.

We use the "select not equal to empty string" thing in the for loop
to avoid `matrix_coturn_turn_external_ip_address` leading to
`matrix_coturn_turn_external_ip_addresses: ['']` leading to
`external-ip=` in the Coturn configuration.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1741