Formatting: Avoid escaping valid XML values in `esc_xml()`.

This change improves the `esc_xml()` function by replacing two `empty()` checks with `isset()` to cover values that are not equal to `''` but still returning `true` when checked with `empty()`, like `'0'`, `0` or `false`. It also updates the related unit tests accordingly.

Props rumpel2116, pbiron.
Fixes #55399.

Built from https://develop.svn.wordpress.org/trunk@53144


git-svn-id: http://core.svn.wordpress.org/trunk@52733 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/formatting.php

@@ -4639,11 +4639,11 @@ EOF;
 	$safe_text = (string) preg_replace_callback(
 		$regex,
 		static function( $matches ) {
-			if ( ! $matches[0] ) {
+			if ( ! isset( $matches[0] ) ) {
 				return '';
 			}
 
-			if ( ! empty( $matches['non_cdata'] ) ) {
+			if ( isset( $matches['non_cdata'] ) ) {
 				// escape HTML entities in the non-CDATA Section.
 				return _wp_specialchars( $matches['non_cdata'], ENT_XML1 );
 			}

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.0-alpha-53143';
+$wp_version = '6.0-alpha-53144';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.