Add unit impl for XcmAssetTransfers trait (#3022)

If an Executor is required in some pallet's config, a mock might be
provided where `Executor = ()`. `()` already implements `ExecuteXcm`,
but not `XcmAssetTransfers`, which is also related. This PR just fixes
that so you can skip having to create a whole xcm configuration, which
is non-trivial right now.

polkadot/xcm/xcm-executor/src/traits/asset_transfer.rs

@@ -85,3 +85,12 @@ pub trait XcmAssetTransfers {
 		}
 	}
 }
+
+impl XcmAssetTransfers for () {
+	type IsReserve = ();
+	type IsTeleporter = ();
+	type AssetTransactor = ();
+	fn determine_for(_: &Asset, _: &Location) -> Result<TransferType, Error> {
+		return Err(Error::UnknownReserve);
+	}
+}