Tests: Do whitespace replacement in `assertDiscardWhitespace()` only when string.

The `assertDiscardWhitespace()` method uses `assertEquals()` under the hood, meaning that in reality any type of actual/expected value should be accepted by the function. Fixed the documentation to reflect that.

At the same time, only strings can contain whitespace differences. So the whitespace replacement should only be done when string values are passed.

This change (a) prevents potential `passing null to non-nullable` errors on PHP 8.1, if either of the inputs would turn out to be `null` and (b) increases tests stability.

Follow-up to [35003], [44902].

Props jrf.
See #53363.
Built from https://develop.svn.wordpress.org/trunk@51698


git-svn-id: http://core.svn.wordpress.org/trunk@51304 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51697';
+$wp_version = '5.9-alpha-51698';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.