Suppress all non-nullable field errors for assignee widget graphql queries

What does this MR do?

This improves the error suppression introduced in !60890 (merged).

Specifically, this MR concerns this part:

Implements a temporary frontend measure for #329750 (closed) until a proper backend fix arrives

• Removes nulls and suppresses the error message Cannot return null for non-nullable field GroupMember.user

Why?

In www-gitlab-com project, the non-nullable error is returned more than once:

Visit https://gitlab.com/gitlab-com/www-gitlab-com/-/boards/836119 and click on any card then check the graphql responses.

Does this MR meet the acceptance criteria?

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