Description

Adding a null check to $hit[_source] for instances when source is not returned.

This scenario happens for instance in our count validation logic. We by design suppress _source which includes documents' content to limit the amount of data needed to transfer as we are only interested in count in that specific scenario.

Checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).
• This change has the fix PRed upstream (if applicable). If not applicable, it has the relevant "// VIP: reason for the discrepancy with upstream" comment in places where the code is discrepant.

Upstream PR - 10up#2992

Steps to Test

1. vip dev-env exec -- wp vip-search health validate-counts
2. No warnings