[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