Fix set cache loop when no attributes exist

Test strictly for false to prevent an endless set cache loop when
you are not using attribute taxonomies or don't have any set.
Prevents `wc_get_attribute_taxonomies` function from calling `wp_cache_set` 
on every page load.

includes/wc-attribute-functions.php

@@ -51,7 +51,7 @@ function wc_get_attribute_taxonomies() {
 	$cache_key   = $prefix . 'attributes';
 	$cache_value = wp_cache_get( $cache_key, 'woocommerce-attributes' );
 
-	if ( $cache_value ) {
+	if ( false !== $cache_value ) {
 		return $cache_value;
 	}