Parents are not shown if only the mother is known

[yannis]

Hi,
Thanks for the great work… really useful to me.

It seems that parents of an individual are not shown in the tree if only the mother is known. This is not true if the father is the only known parent.

You can reproduce it by using the following data.json

[
  {"id":2001, "name":"Rebecca Carter", "fathers":[], "mothers":[1], "gender":"F", "husb":[]},
  {"id":1, "name":"Lydia Ames", "fathers":[], "mothers":[], "gender":"F", "husb":[]}
]

I'll try to fix this and send you a PR, but in the meantime it would be great if you had a quick fix.