Code Modernization: Fix "passing null to non-nullable" deprecation in _mb_substr().

The `_mb_substr()` function expects a string for the `$str` parameter, but does not do input validation. This function contains a `preg_match_all()` which also expects a string type for the given subject (i.e. `$str`). 

Passing `null` to this parameter results in `preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated` notice on PHP 8.1.

To maintain the same behaviour as before, a guard clause is added to bail out early when `$str` is passed as `null`. The outcome will, in that case, only ever be an empty string.

Note: this does mean that the `_mb_substr()` function now has a subtle difference in behaviour compared to the PHP native `mb_substr()` function as the latter ''will'' throw the deprecation notice.

The existing tests already cover this issue.

Follow-up to [17621], [36017], [32364].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51853 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/compat.php

@@ -79,6 +79,10 @@ endif;
  * @return string Extracted substring.
  */
 function _mb_substr( $str, $start, $length = null, $encoding = null ) {
+	if ( null === $str ) {
+		return '';
+	}
+
 	if ( null === $encoding ) {
 		$encoding = get_option( 'blog_charset' );
 	}