When using Client-side Live Preview, array fields are unable to clear
all their rows. This is because `reduceFieldsToValues` sets the array's
value as 0 within form-state when no rows exist, as opposed to an empty
array as one might expect. For now, we can simply handle this data shape
within Live Preview's merge logic. In the future we may want to take to
consider changing the behavior of empty arrays within form-state itself.
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