[ML] Fix Single Metric Viewer chart failing to load if no points during a calendar event

[qn895]

Summary

This PR fixes #128462 where Single Metric Viewer chart fails to load if no points during a calendar event.

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Checklist

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Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
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For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Ideally we should be adding points for the calendar events to the charts - adding points, with the usual rectangular scheduled event marker, at chart aggregation interval during the time span of the calendar event, with y-axis values at 0. Is this possible?

[alvarezmelissa87]

This diff1 will be NaN if previousItem is undefined - is that intended?

[qn895]

This is fine as it's only used for a boolean check

[kibana-ci]

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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
ml	3.3MB	3.3MB	+562.0B

Unknown metric groups

ESLint disabled in files

id	before	after	diff
apm	15	14	-1
securitySolution	69	68	-1
uptime	7	6	-1
total			-3

ESLint disabled line counts

id	before	after	diff
apm	88	85	-3
uptime	49	43	-6
total			-9

Total ESLint disabled count

id	before	after	diff
apm	103	99	-4
securitySolution	511	510	-1
uptime	56	49	-7
total			-12

History

• 💚 Build #34645 succeeded e64960c

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

cc @qn895

[peteharverson]

Closing in favor of #130000.