Relationship field: Fix return data when value exists + Allow -1 as limit

[JoryHogeveen]

Description

Allow -1 (unlimited) as filtered value for the relationship limit

Related GitHub issue(s)

Fixes: #7111

Changelog text for these changes

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.
• My code includes automated tests for PHP and/or JS (if applicable).

[what-the-diff]

PR Summary

• Add Condition to Limit Check
  To ensure smooth operation, we've added a safeguard in the pick.php file. Now it verifies that the 'limit' value isn't equal to -1 before comparing it with the size of a certain set of data. This helps to reduce the likelihood of any errors during this process.

• Enhancing Search Select Results
  We've made improvements to how we handle the results of a search selection in the same pick.php file. The returned search results are now merged into the existing array of results, providing a more comprehensive overview when a search is performed.