Media: Prevent PHP notice in get_avatar().

Follow up fix to [56690].

Props pereirinha, mukesh27, spacedmonkey.
Fixes #58892.


git-svn-id: https://develop.svn.wordpress.org/trunk@56756 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/pluggable.php

@@ -2845,6 +2845,7 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			'loading'       => null,
 			'fetchpriority' => null,
 			'extra_attr'    => '',
+			'decoding'      => null,
 		);
 
 		if ( empty( $args ) ) {