Re: BUG #19857: Stats restore rejects empty arrays, losing range and array stats

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> Looking into this, probably a duplicate of 19842
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It's definitely a duplicate of 19842, but because it would need to be
backported to 18, we'd want to handle this one first, and build
the 19842-stuff on top of that.

I have relaxed the test in stats_check_required_arg() to allow for empty
arrays, which itself creates a burden of checking the number of dimensions
of an array before asking for the number of vals in the 0th dimension,
which needs to be done twice in attribute_statistics_update_internal() and
nowhere else, mostly because there are only two arrays (most_common_values
and most common_freqs) that need to have an equal number of values (MCELEM
has additional values over its freqs, and range_length_hisotgram is paired
with the scalar range_null_frac as a way to save slots, I presume.

Added additional test cases to show that the length agreement requirement
still works on MCV/MCF when one is empty, and that both store empty arrays
without complaint.