Set `allow_blank: false` for the `private_profile` param in User API
See the discussion in !107362 (comment 1226433852) for details.
This is a breaking change for the API, and hence can only be done in 16.0
TODO:
- Remove the TODO items mentioned in
lib/api/users.rb
related to this issue - Add
allow_blank: false
for the API paramprivate_profile