Fix: 'sites' => 'current' doesn't work as expected

[burhandodhy]

Description of the Change

This PR fixes the issue where the 'sites' => 'current' doesn't work after as expected.

Closes #3225

How to test the Change

Run the below query on the multisite and it should only returns the post of current site

		$query = new \WP_Query(
			array(
				's'  => 'site',
				'sites' => 'current',
			)
		);

Changelog Entry

Fixed - 'sites' => 'current' doesn't work

Credits

Props @oscarssanchez @anders-naslund @burhandodhy

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.