KSES: Early-abort in wp_kses_hair() when no attributes exist.

When `wp_kses_hair()` calls into the HTML API to parse an attribute string, it checks if the result might be `null` and returns early, skipping a few minor operations. It could also skip when the returned attribute count is zero.

This patch adds the additional check and early-return.

Developed in: WordPress#10764
Discussed in: https://core.trac.wordpress.org/ticket/63724

Follow-up to [61499].

Props dd32, dmsnell, jonsurrell.
See #63724.


git-svn-id: https://develop.svn.wordpress.org/trunk@61503 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/kses.php

@@ -1624,7 +1624,7 @@ function wp_kses_hair( $attr, $allowed_protocols ) {
 	$processor->next_token();
 
 	$attribute_names = $processor->get_attribute_names_with_prefix( '' );
-	if ( ! isset( $attribute_names ) ) {
+	if ( null === $attribute_names || 0 === count( $attribute_names ) ) {
 		return $attributes;
 	}