[Backport 2.x] Add an _exists_ check to document level monitor queries (#1425)

[jowg-amazon]

Manual backport of #1425 to 2.x

Issue #, if available:

Description of changes:

CheckList:

• [ ] Commits are signed per the DCO using --signoff

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[jowg-amazon]

"org.opensearch.alerting.resthandler.WorkflowRestApiIT.test chained alerts and audit alerts for workflows with query level monitor" test in multi-node test workflow was failing in previous merged PRs

• https://github.com/opensearch-project/alerting/actions/runs/7934795117/job/21666521896?pr=1434
• https://github.com/opensearch-project/alerting/actions/runs/8186232784/job/22384230124?pr=1452