Added edge case handling to aggregation example in docs [skip ci]

See https://github.com/yiisoft/yii2/issues/12491#issuecomment-276593948

docs/guide/db-active-record.md

@@ -1625,7 +1625,7 @@ class Customer extends \yii\db\ActiveRecord
             return null; // this avoid calling a query searching for null primary keys
         }
         
-        return $this->ordersAggregation[0]['counted'];
+        return empty($this->ordersAggregation) ? 0 : $this->ordersAggregation[0]['counted'];
     }
 
     /**