Fix crash on group removal

Summary:

• Add convenience method for counting groups. Makes code more readable/easy to understand than calling rowCount().
• Fix determination of group count for group removal.

Fixes T29545

Test Plan:

• Try to delete last remaining group
  • Option to do so shouldn't appear
  • Even when doing so through other faulty UI (see T29551: [Segmentation] Label tool buttons should be updated whenever the model changes), a message box pops up explaining that it cannot be done
• Try to delete any other group
  • No issues

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T29545

Differential Revision: https://phabricator.mitk.org/D811