Add null check for subclass selectors

Author: sirreal

src/wp-includes/html-api/class-wp-css-selectors.php

@@ -962,9 +962,11 @@ public function matches( WP_HTML_Processor $processor ): bool {
 				return false;
 			}
 		}
-		foreach ( $this->subclass_selectors as $subclass_selector ) {
-			if ( ! $subclass_selector->matches( $processor ) ) {
-				return false;
+		if ( null !== $this->subclass_selectors ) {
+			foreach ( $this->subclass_selectors as $subclass_selector ) {
+				if ( ! $subclass_selector->matches( $processor ) ) {
+					return false;
+				}
 			}
 		}
 		return true;