Ensure matrix_user_verification_service_uvs_access_token is always defined

The playbook tries to avoid such variables which are sometimes defined
and sometimes not. We'd rather not check for `is defined`.

roles/custom/matrix-user-verification-service/defaults/main.yml

@@ -32,7 +32,7 @@ matrix_user_verification_service_systemd_service_name: "{{ matrix_user_verificat
 ## REQUIRED
 
 # Homeserver client API admin token (synapse only)-  Required for the service to verify room membership
-# matrix_user_verification_service_uvs_access_token:
+matrix_user_verification_service_uvs_access_token: ''
 
 # homeserver client api url
 matrix_user_verification_service_uvs_homeserver_url: "{{ matrix_homeserver_container_url }}"