[keyserver] Fix error when changing thread description

Summary:
issue: https://linear.app/comm/issue/ENG-6013/unable-to-update-chat-description
In D9859 sqlUpdate.description = description ?? null; was changed to sqlUpdate.description;. This is probably a mistake, and should be sqlUpdate.description = description;. After this change it is possible to change the chats description again.

Test Plan: Tested that changing chat description works

Reviewers: marcin, ashoat, tomek

Reviewed By: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10189