[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