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