Fix ES client for nil password

What does this MR do and why?

This MR fixes the elasticsearch_config method to address https://gitlab.com/gitlab-org/gitlab/-/issues/368703. Currently if we only set username, password will be set to nil and break the elasticsearch client.

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How to set up and validate locally

1. Enable Advanced Search
2. Set the username with empty password

ApplicationSetting.current.update(elasticsearch_username: 'test', elasticsearch_password: '')

3. Build Elasticsearch client using elasticsearch_config

::Elasticsearch::Client.new(ApplicationSetting.current.elasticsearch_config)

4. It shouldn't raise an exception

MR acceptance checklist

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Related to #368703