[Custom threshold] Fix not showing threshold line in lens preview

[maryam-saeidi]

Closes #171843

Summary

This PR fixes not showing the threshold line in the following case:

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[ghost]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• /oblt-deploy-serverless : Deploy a serverless Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[benakansara]

LGTM

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 626644d

Failed CI Steps

• FTR Configs #56

Test Failures

• [job] [logs] FTR Configs #56 / management advanced settings feature controls settings can be saved with the advancedSettings: ["all"] feature privilege

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
observability	1.1MB	1.1MB	+32.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @maryam-saeidi