[web] Hide "Users in parent channel" header for threads without parent in AddMembersModal

Summary:
The diff is solution for: https://linear.app/comm/issue/ENG-1396/addmembersmodal-on-web-shows-users-in-parent-channel-for-community
It provides additional check if channel (that we want to add users to) has parent channel, and hides "Users in parent channel" label if there is no parent channel

Test Plan: Tested the modal in Genesis and one manually created COMMUNITY_ROOT thread (to achive something similar to Comm channel in production, where the issue appeared) and confirmed, that the header is not displayed and the modal looks good.

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4574