Prevent index out of range in case of empty or malformed filename

Summary:
Prevent index out of range in case of empty or malformed filename

The Akonadi cache on this computer somehow has an external file missing. Without this change, the IMAP agent crashes on every attempt to start it.

Reviewers: KDE PIM, dvratil

Subscribers: knauss, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8968