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.