chore: fix rpc url for arb if no env var (#12320)

Author: grandizzy

crates/test-utils/src/rpc.rs

@@ -188,7 +188,11 @@ fn next_url_inner(is_ws: bool, chain: NamedChain) -> String {
     }
 
     if matches!(chain, Arbitrum) {
-        return env::var("ARBITRUM_RPC").unwrap_or("https://arb1.arbitrum.io/rpc".to_string());
+        let rpc_url = env::var("ARBITRUM_RPC").unwrap_or_default();
+        if !rpc_url.is_empty() {
+            return rpc_url;
+        }
+        return "https://arb1.arbitrum.io/rpc".to_string();
     }
 
     let reth_works = true;