Fix validation and prevent empty "external-ip=" lines in Coturn config

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

roles/matrix-coturn/tasks/validate_config.yml

@@ -6,5 +6,4 @@
       You need to define a required configuration setting (`{{ item }}`) for using Coturn.
   when: "vars[item] == ''"
   with_items:
-    - "matrix_coturn_turn_external_ip_address"
     - "matrix_coturn_turn_static_auth_secret"