[DAR-2228][External] - Fix for NifTI exports containing no polygons

[JBWilkie]

Problem

A change made in DAR-1165 accidentally introduced a bug whereby NifTI exports of files containing no polygons would fail.

Solution

• 1: Only attempt to export polygons if any polygon annotations are present
• 2: Added unit tests to make sure we're calling the polygon to volume & mask to volume functions when appropriate

Changelog

Fixed issue that caused NifTI exports without polygons to fail

[linear[bot]]

DAR-2228 Missing affine or shape when exporting to NifTI