[DAR-2504][External] Generate empty NifTI volumes when no polygons are present

[JBWilkie]

Problem

Currently, if no polygons are present in a NifTI file at the point of export, no file will be generated. This is problematic for our backend exports which seem to hang in these scenarios

Solution

If there are no polygons, export an empty NifTI file

Changelog

Generate an empty NifTI file if no polygons are present for export

[linear[bot]]

DAR-2504 Output a .nii.gz file even for empty NifTI exports