Check if the filename is empty when user wants to save the file

Summary: KEduVocDocument default name is empty and that's preventing user to save the new document when the window closed and KWordQuiz asks if the user want sto save the document.

Test Plan:

1. Open KWordQuiz window, type something in the first column.
2. Click on the close window button or press Ctrl+Q.
3. Click on "Save".
4. The Save dialog should be opened (before the patch, KWQ just did nothing and closed the confirmation window).

Reviewers: KDE Edu

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D20527