Skips over metadata properties with null value.

Perhaps we should be logging these, because I
don't think that this is legal NexSON. But some
are making it into the phylesystem_test repo.
I'll submit an issue there, but this is a
workaround.
just a comment for posterity
Merge pull request #13 from OpenTreeOfLife/skipping-null-prop-vals

Skips over metadata properties with null value.