Use " " instead of "" for the empty key so sh doesn't interpret it as falsy

substrate/client/tests/validator_sets.rs

@@ -102,7 +102,7 @@ async fn validator_set_rotation() {
     ])
     .replace_env(HashMap::from([
       ("RUST_LOG".to_string(), "runtime=debug".to_string()),
-      ("KEY".to_string(), String::new()),
+      ("KEY".to_string(), " ".to_string()),
     ]))
     .set_publish_all_ports(true)
     .set_handle(handle(name))