chore: fix return type in WP_Duotone_Gutenberg::get_selector() (#66695)

Co-authored-by: justlevine <[email protected]>
Co-authored-by: gziolo <[email protected]>

lib/class-wp-duotone-gutenberg.php

@@ -640,7 +640,7 @@ private static function get_global_styles_presets( $sources ) {
 	 *
 	 * @param string $block_name The block name.
 	 *
-	 * @return string The CSS selector or null if there is no support.
+	 * @return ?string The CSS selector or null if there is no support.
 	 */
 	private static function get_selector( $block_name ) {
 		$block_type = WP_Block_Type_Registry::get_instance()->get_registered( $block_name );
@@ -669,6 +669,8 @@ private static function get_selector( $block_name ) {
 			// Regular filter.duotone support uses filter.duotone selectors with fallbacks.
 			return wp_get_block_css_selector( $block_type, array( 'filter', 'duotone' ), true );
 		}
+
+		return null;
 	}
 
 	/**