Fix loading authors in issues list

What does this MR do and why?

This MR filters out null users so that when the backend passes us null users we don't pass them to components that expect a valid user to be present.

Screenshots or screen recordings

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How to set up and validate locally

1. Go to Board issues list
2. Add res.push(null) to this line in your code https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_shared/components/filtered_search_bar/tokens/author_token.vue#L62

This replicates having the backend return null values.

1. Open the authors token and make sure that the list renders.

MR acceptance checklist

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