_path.get_path_collection_extents, when applied to a collections with offsets=[] treats the offsets as if they were [0, 0](this behavior is used in path.get_paths_extents).

This interpretation of an empty collection is error-prone, and caused #1490. Hence it would be desirable to decide what should path.get_path_collection_extents return when given an empty collection, and have path.get_path_extents not rely on current behavior.

This issue is created to not lose track of the bug, while closing #1490 and merging #1497.