Fix issue where data hole exists for Preview API

[yizheliu-amazon]

Issue #, if available:

Description of changes:
Fix issue where data hole exists for Preview API. For now, we only get cold start samples with non-zero doc, and we add samples to corresponding date range by index of cold start samples. This can cause issue because when source data has no doc in periods of time, size of featureSamples is less than sampleRanges, then only the some beginning date ranges of sampleRanges have data returned.

Fix is simple: just include zero count bucket in the response from ES.

Example: data only exists in source starting from 11-13-2020
Before:

After:

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[yizheliu-amazon]

Checks are failing because set-env is disabled. Have run gradle build locally to make sure all tests pass

[kaituo]

Checks are failing because set-env is disabled. Have run gradle build locally to make sure all tests pass

Weicong's PR fixed it.