Fix crash when loading multilabel nifty segmentations
Summary: Add nullptr checks for legacy, binary segmentations
Test Plan: manual testing according to the task description
Reviewers: kislinsk, floca, O1 MITK Reviewer Group I
Reviewed By: floca, O1 MITK Reviewer Group I
Subscribers: kalali, floca, kislinsk
Maniphest Tasks: T29388
Differential Revision: https://phabricator.mitk.org/D757