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

substrate/client/tests/common/mod.rs

@@ -32,7 +32,7 @@ macro_rules! serai_test {
         .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)