Clear preview segmentation in fast marching 3D

Summary:
Update also if the seed points are empty

This will automatically remove the preview segmentation after all seed points have been removed.
This is required for the "Clear"-button to work correctly.

Test Plan: manual UI testing

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Subscribers: kalali, floca

Maniphest Tasks: T27870

Differential Revision: https://phabricator.mitk.org/D446