Export: Fix fatal error when passing null to wxr_cdata() by casting passed value to string.

This ensures that `wp_is_valid_utf8()` does not cause a type error since it only accepts strings. 

Developed in https://github.com/WordPress/wordpress-develop/pull/10595

Follow-up to [60630].

Props hbhalodia, westonruter, desrosj, albigdd, jorbin.
See #38044.
Fixes #64347.

Built from https://develop.svn.wordpress.org/trunk@61405


git-svn-id: http://core.svn.wordpress.org/trunk@60717 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/export.php

@@ -239,10 +239,12 @@ function export_wp( $args = array() ) {
 	 *
 	 * @since 2.1.0
 	 *
-	 * @param string $str String to wrap in XML CDATA tag.
+	 * @param string|null $str String to wrap in XML CDATA tag. May be null.
 	 * @return string
 	 */
 	function wxr_cdata( $str ) {
+		$str = (string) $str;
+
 		if ( ! wp_is_valid_utf8( $str ) ) {
 			$str = utf8_encode( $str );
 		}

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-alpha-61404';
+$wp_version = '7.0-alpha-61405';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.