Notatrix throws away relations in the input without heads

[ftyers]

 # sent_id = Frog_Story:69
 # text = Wan n telpukatl kwika oksipa se chihchikichi.
 # text[orig] = wan n teelpukatl kwiika oksipa see chihchikichi~.
 # text[spa] = Y el niño lleva otra vez una chiquita.
 1  Wan _   _   _   _   _   _   _   _
-2  n   in  DET _   _   _   det _   _
+2  n   in  DET _   _   _   _   _   _
 3  telpukatl   telpukatl   NOUN    _   _   _   _   _   _
 4  kwika   _   _   _   _   _   _   _   _
 5-6    oksipa  _   _   _   _   _   _   _   _
-5  o   o   AUX _   _   _   aux _   _
+5  o   o   AUX _   _   _   _   _   _
 6  ksipa   _   VERB    _   _   _   _   _   _
 7  se  _   _   _   _   _   _   _   _
 8  chihchikichi    _   _   _   _   _   _   _   _
-9  .   .   PUNCT   _   _   _   punct   _   _
+9  .   .   PUNCT   _   _   _   _   _   _

[jonorthwash]

Expected behaviour: keep the deprel even though a dependency arc can't be drawn.

This is probably a minor design flaw in notatrix.

[jonorthwash]

Duplicate of #335.