[web] Allow creating subchannels with no other users

Summary:
issue: [[https://linear.app/comm/issue/ENG-8811/[web]-dont-require-members-when-creating-subchannel | ENG-8811]]
We want to match native and allow creating subchannels with no other users

Test Plan:
Tested that it is possible to create a subchannel with no other users
Tested that the created robotext makes sense:
"you created this channel as a child of delete thread with the name "m h""

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12726