Fix nullptr dereference when replying to emails without a CC header

Summary:
This is a regression from 7763a47b49 / D24624. The previous code used
cc() rather than cc(false) as well here, which avoids the problem (despite
maybe not being entirely ideal).

Reviewers: gjditchfield, knauss, mlaurent

Reviewed By: gjditchfield, mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D24891