[Lens] [@kbn/tinymath] Add new defaults function

[dej611]

Summary

Closes #140829

This PR introduces a new defaults function, useful to gracefully fallback from missing values.

I'm not really super happy about the doc here, as there's no other mention of null in any other tinymath function in the doc. 🤔

The new function has been documented also on the Lens Formula side.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios

Risk Matrix

Delete this section if it is not applicable to this PR.

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.
See more potential risk examples

For maintainers

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

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[flash1293]

LGTM, tested and works as expected

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 5be30d0

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
canvas	1152	1153	+1
expressions	172	173	+1
lens	922	923	+1
total			+3

Async chunks

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

id	before	after	diff
canvas	1.0MB	1.0MB	+176.0B
expressions	25.6KB	25.7KB	+170.0B
lens	1.3MB	1.3MB	+631.0B
total			+977.0B

History

• 💚 Build #77864 succeeded f9b6a97
• 💚 Build #77413 succeeded a42ab15
• 💚 Build #77241 succeeded 8bc2855
• 💚 Build #76846 succeeded 8250711
• 💔 Build #76449 failed b1d2f5f

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@elasticmachine merge upstream