[Segmentation] Respect null labels but do not try to save properties for them
[Segmentation] Respect null labels but do not try to save properties for them

Summary: Respect null labels but do not try to save properties for them. Fixes T28588.

Test Plan: Manually saved and loaded labelset presets for affected images.

Reviewers: O1 MITK Reviewer Group I, floca

Reviewed By: O1 MITK Reviewer Group I, floca

Maniphest Tasks: T28588

Differential Revision: https://phabricator.mitk.org/D513

MITK/MITK,MITK/MITK,MITK/MITK,MITK/MITK,MITK/MITK,MITK/MITK