[keyserver] Don't specify name for new PRIVATE threads

Summary:
Now that we display PRIVATE threads without names with the ENS-resolved username of the member, we can skip specifying the name of the thread.

Depends on D7453

Test Plan: In combination with the preceding diff, I tested this by deleting and recreating my ashoat.eth account in my local web environment, and confirming that my new PRIVATE thread was displayed with the name "ashoat.eth"

Reviewers: tomek, atul

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D7454