[resources/maildir] Don't create junk files

Summary:
As user types path inside configuration dialog, it gets checked to display
the comment whether it's a valid maildir or not. Under the hood, it calls
maildir.isValid(createMissingFolders=false). There was a codepath, however,
where createMissingFolders is not provided (and the default value is true),
thus creating cur/new/tmp folders all over the place as user types the path.

Test Plan:

1. Try to create maildir inside /tmp/dummy
2. Observe cur/new/tmp folders being created everywhere inside /tmp/*
3. After applying the patch, they don't

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D27906