Failure on loading some .spine/.outline files

Some tracks in Choreography files cause a bug in DecoratedLarva with: ArgumentError: collection must be non-empty

The path attribute is supposedly empty, while the files actually define many time steps.

Steps to reproduce: Download these sample files, unpack them and open either the .spine or .outline file with larvaeditor.

Proposal: Catch the errors and skip the problematic tracks.

A same problem may occur at the backend level. We'll see.

EDIT: Exact same error in the backend, although it's Python code: ValueError: need at least one array to stack