Reuse unmodified empty document instead of closing it

Summary:

• In VI-Mode, using :e filename or :tabe filename in the empty unmodified initial document makes Kate crash. The caller is deleted by closing the document.
• Reusing the already opened document avoids the problem.

Test Plan: - use :e filename in VI-Mode from a fresh kate instance

Reviewers: Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann

Tags: Kate

Differential Revision: https://phabricator.kde.org/D8713