Don't call append_to_selector() when selector is likely empty.

lib/class-wp-theme-json-gutenberg.php

@@ -1556,10 +1556,13 @@ protected static function compute_preset_classes( $settings, $selector, $origins
 			$slugs = static::get_settings_slugs( $settings, $preset_metadata, $origins );
 			foreach ( $preset_metadata['classes'] as $class => $property ) {
 				foreach ( $slugs as $slug ) {
-					$css_var     = static::replace_slug_in_string( $preset_metadata['css_vars'], $slug );
-					$class_name  = static::replace_slug_in_string( $class, $slug );
-					$stylesheet .= static::to_ruleset(
-						static::append_to_selector( $selector, $class_name ),
+					$css_var    = static::replace_slug_in_string( $preset_metadata['css_vars'], $slug );
+					$class_name = static::replace_slug_in_string( $class, $slug );
+
+					// $selector is often empty, so we can save ourselves the `append_to_selector()` call then.
+					$new_selector = '' === $selector ? $class_name : static::append_to_selector( $selector, $class_name );
+					$stylesheet  .= static::to_ruleset(
+						$new_selector,
 						array(
 							array(
 								'name'  => $property,