Unexpected values for branchLength* for trees with no branch lengths

This is what I get from an otindex response for a tree that has no edge lengths:
```
...
"ot:branchLengthDescription": "",
"ot:branchLengthMode": "ot:undefined",
...
```
It _seems_ like the tree might(?) have edge lengths, but the flavour has just not been specified. I was expecting something like `"absent"` or `"NA"` (although the former is more explicit, I think).