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

Fixes #45739.

Built from https://develop.svn.wordpress.org/trunk@45619


git-svn-id: http://core.svn.wordpress.org/trunk@45430 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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 ),

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45618';
+$wp_version = '5.3-alpha-45619';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.