use empty to check to see if attributes for variations exist

Closes #8959

includes/admin/meta-boxes/class-wc-meta-box-product-data.php

@@ -605,7 +605,7 @@ class WC_Meta_Box_Product_Data {
 
 		if ( $attributes ) {
 			foreach ( $attributes as $attribute ) {
-				if ( isset( $attribute['is_variation'] ) ) {
+				if ( ! empty( $attribute['is_variation'] ) ) {
 					$variation_attribute_found = true;
 					break;
 				}