Only advertise 3478/udp (STUN/TURN) if Coturn actually enables it

Related to ac26cc1cb0 which disabled
`3478/udp` by default.

Without this patch, homeservers continued to advertise the `3478/udp` port,
even though it was no longer exposed. While clients should handle that
gracefully (by falling back to TCP, etc.), it seems like Element either
doesn't do that or it doesn't support TCP (which this patch won't fix).

In any case, it's better to only announce what is actually supported/exposed.

roles/custom/matrix-coturn/vars/main.yml

@@ -9,6 +9,9 @@ matrix_coturn_turn_uris: |-
     +
     ([
       'turn:' + matrix_server_fqn_matrix + '?transport=udp',
+    ] if (matrix_coturn_container_stun_plain_host_bind_port_udp != '' or matrix_coturn_container_network == 'host')  else [])
+    +
+    ([
       'turn:' + matrix_server_fqn_matrix + '?transport=tcp',
-    ])
+    ] if (matrix_coturn_container_stun_plain_host_bind_port_tcp != '' or matrix_coturn_container_network == 'host')  else [])
   }}