Block Editor: Fix PHP warning when loading editor styles while in RTL.

In RTL languages, WordPress adds `style-editor-rtl.css` editor styles to the global `$editor_styles`.
This patch ignores handling these styles if the file is not preset.

Also, clarify the docs for the return value of the `block_version` function.

Props mostafa.s1990, desrosj, mukesh27.

Merges [43923] and [43924] to trunk.

Fixes #45288, #45342.
Built from https://develop.svn.wordpress.org/trunk@44270


git-svn-id: http://core.svn.wordpress.org/trunk@44100 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/edit-form-blocks.php

@@ -198,12 +198,14 @@ if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) {
 			}
 		} else {
 			$file = get_theme_file_path( $style );
+			if ( file_exists( $file ) ) {
 				$styles[] = array(
-				'css'     => file_get_contents( get_theme_file_path( $style ) ),
+					'css'     => file_get_contents( $file ),
 					'baseURL' => get_theme_file_uri( $style ),
 				);
 			}
 		}
+	}
 }
 
 // Image sizes.

wp-includes/blocks.php

@@ -283,7 +283,7 @@ function _restore_wpautop_hook( $content ) {
  * @since 5.0.0
  *
  * @param string $content Content to test.
- * @return int The block format version.
+ * @return int The block format version is 1 if the content contains one or more blocks, 0 otherwise.
  */
 function block_version( $content ) {
 	return has_blocks( $content ) ? 1 : 0;

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44269';
+$wp_version = '5.1-alpha-44270';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.