crash on linux calling layout_from_lsh_forest with 0-size LSHForest

[doublethefish]

Repo:

1. build with debug symbol on linux (I used the python:3.9.10-bullseye docker image)
2. call into layout_from_lsh_forest with a zero-size LSHForest
3. observe a crash when de-referencing an invalid end-iterator

The cause seems to be float max_x = *max_element(x.begin(), x.end()); where we dereference the invalid end-iterator returned by max_element when called with an empty container. This should crash more often, but although I've not looked into it properly, but because we dereference into a float we probably just get garbage data on the other platforms.

I have a fix for this code path and MSTFromLSHForest where we return an tuple of empty objects for an empty forest. The fix is on the development branch.