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.

git-svn-id: https://develop.svn.wordpress.org/trunk@44270 602fd350-edb4-49c9-b593-d223f7449a82

src/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;