D20016: Don't set a null completer on a non-editable combobox

[Elvis Angelaccio]

elvisangelaccio added a reviewer: dfaure.

REPOSITORY
  R284 KCompletion

REVISION DETAIL
  https://phabricator.kde.org/D20016

To: elvisangelaccio, dfaure
Cc: kde-frameworks-devel, michaelh, ngraham, bruns