Fix QCompleter::activated(QModelIndex)

When falling back to the completion prefix, make sure to also pass an
invalid index to activated().

Change-Id: I6b282a01c95492466890632b77837bcc96eb038a
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>