ui: info about untracked files should only display if untracked files exist.

[FlorianHeigl]

since the info texts are in list format one thinks for a moment there's a untracked files.
Then on almost the same indentation there' the textual info There are no untracked files to list.

$ gl status 
On branch master, repo-directory //

Tracked files with modifications:
  ➜ these will be automatically considered for commit
  ➜ use gl untrack f if you don't want to track changes to file f
  ➜ if file f was committed before, use gl checkout f to discard local changes

    roles/check_mk-pfsense/tasks/main.yml


Untracked files:
  ➜ these won't be considered for commit
  ➜ use gl track f if you want to track changes to file f

    There are no untracked files to list

I don't know which is the best approach to display here.

• it might be useful that the info is further indented than the data.
• it itches that in the positive case there's just a list without prefix or comments, but in the negative case we have a human readable repetetion of the section header. I would trim it to 'No untracked files' or 'No such files' or just 'None'