Should UniFrac ever return NaN?

[ElDeveloper]

There's a few cases where this happens:

• The counts table is all full of zeros (using weighted normalized, weighted unnormalized and unweighted unifrac).
• The counts table is empty (using weighted normalized, weighted unnormalized and unweighted unifrac).

These tests were not included, but were present in skbio's test_unweighted_unifrac_zero_counts, test_weighted_unifrac_zero_counts_normalized, and test_weighted_unifrac_zero_counts.