[Backport 2.x] Defaults anomaly grade to 0 if negative

[joshpalis]

Description

Manual Backport of #976 (without the import changes since these were already handled by #974 )

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[joshpalis]

Failing again due to stale snapshots :

Could not determine the dependencies of task ':integTest'.
You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.
> Can't get https://ci.opensearch.org/ci/dbc/distribution-build-opensearch/2.10.0/latest/linux/x64/tar/builds/opensearch/plugins/opensearch-job-scheduler-2.10.0.0.zip to D:\a\anomaly-detection\anomaly-detection\src\test\resources\job-scheduler\opensearch-job-scheduler-2.10.0.0.zip