Tests: Remove nested empty directories in WP_UnitTestCase_Base::rmdir().

Includes:
* Checking if the directory exists and returning early otherwise.
* Removing a redundant `rmdir()` call in `clean_dirsize_cache()` tests.

Follow-up to [49744], [54300].

See #55652.

git-svn-id: https://develop.svn.wordpress.org/trunk@54303 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/multisite/cleanDirsizeCache.php

@@ -295,8 +295,6 @@ if ( is_multisite() ) :
 
 			// Cleanup.
 			$this->remove_added_uploads();
-			rmdir( $upload_dir['basedir'] . '/2/1' );
-
 			restore_current_blog();
 		}