Merge branch 'ak/add-i-empty-candidates' into maint

The interactive "show a list and let the user choose from it"
interface "add -i" used showed and prompted to the user even when
the candidate list was empty, against which the only "choice" the
user could have made was to choose nothing.

* ak/add-i-empty-candidates:
  add -i: return from list_and_choose if there is no candidate