Summary:
Drop one usage of QSet::fromList(), and use QList::removeDuplicates()
instead, assuming the conversion for QList to QSet was to remove
duplicates.

Return early if nameValue is empty, this also guards against accesing
an out of bounds index.

Use QList::at() instead of [], to prevent detaching.

Test Plan: using sasl still works

Reviewers: #konversation, psn, hein, mlaurent

Reviewed By: #konversation, psn

Differential Revision: https://phabricator.kde.org/D27359