Editor: Fix render_duotone_support() to be compatible with enhanced pagination.

Some blocks do not have content. For duotone support, blocks without content still need to run through the `render_duotone_support()` to render their duotone CSS.

This fix makes the duotone compatible with the enhanced pagination (introduced in 6.4.0) by making sure that the CSS is always on the page, even when the posts have no featured image. It also prevents the duotone from interfering with other blocks using `wp_unique_id()`.

References:
* [https://github.com/WordPress/gutenberg/pull/55415 Gutenberg PR 55415]

Follow-up to [56226].

Props cbravobernal, luisherranz, hellofromTonya, isabel_brison, jorbin.
Reviewed by costdev.
Merges [56991] and [56996] to the 6.4 branch.
Fixes #59694.
Built from https://develop.svn.wordpress.org/branches/6.4@56997


git-svn-id: http://core.svn.wordpress.org/branches/6.4@56508 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-duotone.php

@@ -1074,7 +1074,7 @@ class WP_Duotone {
 	 * @return string Filtered block content.
 	 */
 	public static function render_duotone_support( $block_content, $block, $wp_block ) {
-		if ( empty( $block_content ) || ! $block['blockName'] ) {
+		if ( ! $block['blockName'] ) {
 			return $block_content;
 		}
 		$duotone_selector = self::get_selector( $wp_block->block_type );

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-RC1-56995';
+$wp_version = '6.4-RC1-56997';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.