Block Editor: Avoid a PHP warning when a theme adds an editor style with an empty filename.

Fixes #45739.


git-svn-id: https://develop.svn.wordpress.org/trunk@45619 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-blocks.php

@@ -192,7 +192,7 @@ if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) {
 			}
 		} else {
 			$file = get_theme_file_path( $style );
-			if ( file_exists( $file ) ) {
+			if ( is_file( $file ) ) {
 				$styles[] = array(
 					'css'     => file_get_contents( $file ),
 					'baseURL' => get_theme_file_uri( $style ),