Fix warning for case of non-existing 'post_type' key in query vars

[econscript]

Description

This pulls a fix made upstream: 10up#4105

Silences a noisy Undefined array key "post_type" PHP warning whenever a query is made without the post_type parameter in the query_vars array.

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.

Steps to Test

[econscript]

I had to pull in this upstream commit to fix the Unit tests: 10up#2984
This changes the ordering to post__in from the default date (post_date) apparently this is an intermittent test failure.

The rest of this method was updated in this PR: https://github.com/Automattic/ElasticPress/pull/228/changes