[Segmentation/IO] Fix several crashes with empty groups

Summary:

• Make LabelSetImage::GetLabelsInGroup() robust against empty group scenarios
• Make interpolation robust against multiple new empty groups at once

Fixes T29573

Test Plan:

• Create segmentation
• Delete last label from Group 0
• Save empty segmentation -> no crash anymore
• Load empty segmentation -> segmentation contains an empty Group 0
• Save project -> no crash anymore
• Load project -> project contains segmentation with empty Group 0

Furthermore open F2658792 to fiddle around with several segmentations that have different empty groups.

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T29573

Differential Revision: https://phabricator.mitk.org/D828