[Bug #1202] Managed division by zero calculating template averages

It's valid to return a 0 value when the the template has never been asigned, so
there are 0 items to calculate the average used time.

Based in previous patch by Lorenzo Tilve Álvaro <ltilve@igalia.com>:
24b0cf6db5

FEA: ItEr75S04BugFixing

navalplanner-business/src/main/java/org/navalplanner/business/orders/daos/OrderElementDAO.java

@@ -291,7 +291,8 @@ public class OrderElementDAO extends IntegrationEntityDAO<OrderElement>
     public EffortDuration calculateAverageWorkedHours(
             final List<OrderElement> list) {
         EffortDuration sum = sumWorkedHours(list);
-        return EffortDuration.average(sum, list.size());
+        return (list.size() == 0) ? EffortDuration.zero() : EffortDuration
+                .average(sum, list.size());
     }
 
     private BigDecimal average(BigDecimal divisor, BigDecimal sum) {