Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/JoinConditionHandle.java
* Each join condition has following properties:
 * 
 * <p>
 * <dl>
 * <dt><strong>Join Type </strong></dt>
 * <dd>the join type of the join condition which can be inner join, left out
 * join and right out join.</dd>
 * </dl>
 * <p>
 * 
 * <p>
 * <dl>
 * <dt><strong>Operator </strong></dt>
 * <dd>the join condition's comparison operator</dd>
 * </dl>
 * <p>
 * 
 * <p>
 * <dl>
 * <dt><strong>Left Dataset </strong></dt>
 * <dd>the left dataset of the join condition.</dd>
 * </dl>
 * <p>
 * 
 * <p>
 * <dl>
 * <dt><strong>Right Dataset </strong></dt>
 * <dd>the right dataset of the join condition.</dd>
 * </dl>
 * <p>