Set a default gas policy (#2830)

### Description

When no gas enforcement policy is set, by default, we should apply None

---------

Co-authored-by: Trevor Porter <trkporter@ucdavis.edu>

rust/agents/relayer/src/msg/gas_payment/mod.rs

@@ -44,6 +44,8 @@ pub struct GasPaymentEnforcer {
 }
 
 impl GasPaymentEnforcer {
+    /// Note that `policy_configs` should not be empty. In the settings,
+    /// a default of vec![GasPaymentEnforcementConf::default()] is used.
     pub fn new(
         policy_configs: impl IntoIterator<Item = GasPaymentEnforcementConf>,
         db: HyperlaneRocksDB,

rust/agents/relayer/src/settings/mod.rs

@@ -187,7 +187,7 @@ impl FromRawConf<RawRelayerSettings> for RelayerSettings {
                     matching_list,
                 })
             }).collect_vec()
-        }).unwrap_or_default();
+        }).unwrap_or_else(|_| vec![GasPaymentEnforcementConf::default()]);
 
         let whitelist = p
             .chain(&mut err)