Removed a GraphQL error suppression on assignees dropdown

What does this MR do?

Previously, due to #329750 (closed) we were filtering null records on frontend and suppressing a GraphQL error about nonnullable field being null.

After the backend resolver changes we ctill need to filter nulls from node.user but as the field is nullable, we don't need to suppress an error anymore.

Screenshots (strongly suggested)

No visual changes

Does this MR meet the acceptance criteria?

Conformity

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Security

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