Fix of broken writer / reader for htg
The recent API change with mask being set to nullptr
rather than an
empty vtkBitArray
broke readers and writers. There was also a few bugs
unrelated to this API change that are addressed.
The new test to ensure that those elements don't break again is half disabled, as there is odd behavior when writing masked htgs. This will be looked at more carefully later.
Addresses paraview/paraview#19973 (closed)
Edited by Yohann Bearzi (Kitware)