Fix new terms enum API when field meta is not passed for autocomplete value suggestions

[dej611]

Summary

Related to #100174

In Lens Formula we use the KQL autocomplete suggestion system via the data.autocomplete plugin.
Because Lens remaps the indexpattern object structure in its own format, the toSpec() method used to provide field metadata to the autocomplete endpoint is not available, and the only information passed are indexpattern and fieldName.
The Formula suggestion system started returning HTTP 500 errors and the root problem was traced back to #100174 .

The new terms enum API introduced the wrong fallback in case of no field provided, and this PR makes it use the fieldName one (as for the terms agg).

I've added also basic unit tests on both functions to cover the absence of field metadata.

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
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• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
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For maintainers

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

[streamich]

LGTM, approving to unblock this fix, would be great if somebody from data plugin would still give it a look. @elastic/kibana-app-services

[dej611]

@elasticmachine merge upstream

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• 💚 Build #136043 succeeded 3e3222f

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💚 Backport successful

Status	Branch	Result
✅	7.14
✅	7.x

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