Fix maintain member for Advanced Search

What does this MR do and why?

This MR fixes

Gitlab::Elastic::DocumentReference::InvalidError
Don't know how to serialize NilClass

The cause for that is Member does not always have an attached user. When we're trying to send nil to ::Elastic::ProcessBookkeepingService.track! it fails.

Sentry: https://sentry.gitlab.net/gitlab/gitlabcom/issues/4073188/?query=is%3Aunresolved%20Gitlab%3A%3AElastic%3A%3ADocumentReference%3A%3AInvalidError

Kibana: https://log.gprd.gitlab.net/app/discover#/?_g=h@e9494ca&_a=h@5f4b352

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