Disable segmentation tools if segmentation is empty

Summary: Fixes T29602

Test Plan:

• Try to activate any tools when the segmentation is empty (shouldn't work)
• Add at least a single label and activate any tool (should work)

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T29602

Differential Revision: https://phabricator.mitk.org/D833