TRAC 53635 | PHP 8.1: convert_invalid_entities() - fix handling of null

Fixes `strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated` notices on PHP 8.1.

Impact: 38 of the pre-existing tests are affected by this issue.

Author: jrfnl

src/wp-includes/formatting.php

@@ -2396,6 +2396,10 @@ function convert_chars( $content, $deprecated = '' ) {
  * @return string Converted string.
  */
 function convert_invalid_entities( $content ) {
+	if ( ! is_string( $content ) || '' === $content ) {
+		return $content;
+	}
+
 	$wp_htmltranswinuni = array(
 		'€' => '€', // The Euro sign.
 		'' => '',