Search: Fix fatal when offloading user queries on WP 6.1

[rebeccahum]

Description

A fatal occurs when attempting to offload a user query with the default fields on WP 6.1:

Adapted from 10up/ElasticPress#3109

Changelog Description

Plugin Updated: Enterprise Search

Fix fatal when offloading user queries on WP 6.1 with the default fields

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 uses a rollout method to ease with deployment (if applicable - especially for large scale actions that require writes).
• This change has relevant documentation additions / updates (if applicable).
• I've created a changelog description that aligns with the provided examples.

Steps to Test

1. Add the snippet:

 add_action( 'init', function() {
    $args = array (
        's'     => 'k',
        'ep_integrate' => true,
    );

    $authors = new WP_User_Query($args);

 });

2. Observe fatal in description
3. pull in PR with submodule update
4. Observe no more fatal and query being offloaded correctly

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Merging #4049 (f93af4f) into develop (4f50274) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #4049   +/-   ##
==========================================
  Coverage      32.23%   32.23%           
  Complexity      3789     3789           
==========================================
  Files            229      229           
  Lines          16965    16965           
==========================================
  Hits            5469     5469           
  Misses         11496    11496           

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[rinatkhaziev]

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