chg: catch empty ``docker-compose.conf`` with meaning-full error message.

bin/compose

@@ -1544,6 +1544,10 @@ launch_docker_compose() {
         _config_merge "$docker_compose_dir/docker-compose.yml" "$state_tmpdir/to-merge-in-docker-compose.yml" || return 1
     fi
 
+    if [ -z "$(echo $(cat "$docker_compose_dir/docker-compose.yml"))" ]; then
+        die "Unexpected empty docker-compose."
+    fi
+
     ## XXXvlab: could be more specific and only link the needed charms
     ln -sf "$CHARM_STORE/"* "$docker_compose_dir/"
     mkdir "$docker_compose_dir/.data"