Media: Use consistent method for instantiating an attachment author object in Media Library.

Previously, attachments without an author could cause a PHP fatal error due to calling the `::exists()` method on a `false` value.

Props antpb, carloscastilloadhoc, hellofromTonya, garrett-eclipse.
Fixes #52030.

git-svn-id: https://develop.svn.wordpress.org/trunk@49979 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -3929,6 +3929,7 @@ function wp_prepare_attachment_for_js( $attachment ) {
 	);
 
 	$author = new WP_User( $attachment->post_author );
+
 	if ( $author->exists() ) {
 		$author_name            = $author->display_name ? $author->display_name : $author->nickname;
 		$response['authorName'] = html_entity_decode( $author_name, ENT_QUOTES, get_bloginfo( 'charset' ) );