fix: avoid PDF sorting error on negative coords (#1361)
The default sorting algorithm for PDF's, "xycut," would cause an error
when partitioning a document if Y coordinate points were negative. This
change checks for that condition (or more broadly, any negative
coordinates) and falls back to the "basic" sort if that is the case.
This PR does not address the underlying issue of "bad points" which
still should be investigated. However, the sorting code should be less
brittle to unexpected bounding boxes in the first case.
Resolves: https://github.com/Unstructured-IO/unstructured/issues/1296
