[lib] Fix invite links validation

Summary:
issue: ENG-8170
Invite links were failing validation, because union was handled incorrectly

This addresses the issue, but I still have to review other tcomb types ENG-8179

Test Plan: Tested that invite links work correclty now

Reviewers: tomek, ashoat, kamil

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12199