Check passed in variables are set

#9983

templates/loop/add-to-cart.php

@@ -24,10 +24,10 @@ global $product;
 echo apply_filters( 'woocommerce_loop_add_to_cart_link',
 	sprintf( '<a rel="nofollow" href="%s" data-quantity="%s" data-product_id="%s" data-product_sku="%s" class="%s">%s</a>',
 		esc_url( $product->add_to_cart_url() ),
-		esc_attr( $quantity ),
+		esc_attr( isset( $quantity ) ? $quantity : 1 ),
 		esc_attr( $product->id ),
 		esc_attr( $product->get_sku() ),
-		esc_attr( $class ),
+		esc_attr( isset( $class ) ? $class : 'button' ),
 		esc_html( $product->add_to_cart_text() )
 	),
 $product );