[stats]: fix panic for empty table stats with one root chunk

[max-hoffman]

There's one root chunk for an empty table that needs special handling because there is no histogram bucket for it.

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
99.999562 to 99.999562

version	result	total
732127d	not ok	26
732127d	ok	5937431

version	total_tests
732127d	5937457

correctness_percentage
99.999562

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
99.999562 to 99.999562

version	result	total
9649a5b	not ok	26
9649a5b	ok	5937431

version	total_tests
9649a5b	5937457

correctness_percentage
99.999562

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
99.999562 to 99.999562

version	result	total
aace986	not ok	26
aace986	ok	5937431

version	total_tests
aace986	5937457

correctness_percentage
99.999562