Fix shipping rate 0 without shipping=

library/rules_shipping_framework.php

@@ -515,7 +515,7 @@ class ShippingRule {
 		/* In the advanced version, all conditions and costs can be given as a full mathematical expression */
 		/* Both versions create an expression tree, which can be easily evaluated in evaluateTerm */
 		$rulepart = trim($rulepart);
-		if (empty($rulepart)) return;
+		if (!isset($rulepart) || $rulepart==='') return;
 
 		
 		// Special-case the name assignment, where we don't want to interpret the value as an arithmetic expression!