Do not execute additional databases creation code if not necessary

The tasks in `create_additional_databases.yml` will likely
ensure `matrix-postgres.service` is started, etc.

If no additional databases are defined, we'd rather not execute that
file and all these tasks that it may do in the future.

roles/matrix-postgres/tasks/setup_postgres.yml

@@ -118,7 +118,7 @@
     apply:
       tags:
         - always
-  when: "matrix_postgres_enabled|bool"
+  when: "matrix_postgres_enabled|bool and matrix_postgres_additional_databases|length > 0"
 
 #
 # Tasks related to getting rid of the internal postgres server (if it was previously enabled)