Update graphql schema to know that community logo is nullable

[henriquecbuss]

On cambiatus/backend#281, we made community logos nullable on graphql. We should update the frontend accordingly. If a community doesn't have a logo, we should use https://cambiatus-uploads.s3.amazonaws.com/cambiatus-uploads/community_2.png by default, as discussed here: https://cambiatus.slack.com/archives/CB0UA5SBE/p1660328600240529