empty string if feature not activated

[github-actions[bot]]

Please complete the following tasks before merging this PR:

• [x] update dependencies (cargo update + check cargo outdated)
• [x] update licenses folder
  • clean current licenses
  • run fetch_licenses.py
  • commit result
  • check test output if licenses are up to date
• [x] update Changelog.md, so all included changes appear under the section
• [x] update zugkontrolle.desktop