Add unit impl for XcmAssetTransfers trait

[franciscoaguirre]

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.

[franciscoaguirre]

bot fmt

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