Fix panic using SortFastShuffle (unreleased new feaure) to encode a struct with no fields.

[benluddy]

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[benluddy]

@fxamacker Thanks for your work preparing the upcoming release. While exercising it today, I discovered that I'd missed this edge case.

[fxamacker]

Nice! Thanks for catching this before SortFastShuffle feature got released! 👍