[Index Template] Fix previewing data streams in template editor

[Dosant]

Summary

fix #139925

Currently, when you create a data stream template and try to use the "preview template" functionality, you'll simulate an index template without a data stream configuration because the data stream is configured using an empty object {} and existing code strips empty objects from the template during simulation:

kibana/x-pack/plugins/index_management/public/application/components/index_templates/simulate_template/simulate_template.tsx

Line 38 in bc2a181

const indexTemplate = serializeTemplate(stripEmptyFields(template) as TemplateDeserialized);

You can see this that by starting creating a data stream:

And then use "preview" on the next step. See there is no data_stream:{} in the simulate request:

This is problematic in cases where a configurion is valid for a data stream template, but invalid for a regular index template. I bumped into this while testing new time series streams in #138748

The fix makes simulation code to remove only empty strings, but leave empty objects untouched.
Please check if this is OK. Looks like all other places which pre-process template are already removing only empty strings. For example:

kibana/x-pack/plugins/index_management/public/application/components/template_form/template_form.tsx

Lines 228 to 230 in bc2a181

	stripEmptyFields(template, {
	types: ['string'],
	}) as TemplateDeserialized

A safer alternative could be to make handling specific for data_stream key

[kibana-ci]

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• Commit: f62d181

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
indexManagement	522.7KB	522.7KB	+59.0B

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[yuliacech]

Thanks a lot for fixing this, @Dosant!
Changes LGTM, tested locally 👍