Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/JoinConditionHandle.java
* Returns left expression value this condition.
	 * 
	 * @return the left expression value
	 */
	public String getLeftExpression( )
	{
		return getStringProperty( JoinCondition.LEFT_EXPRESSION_MEMBER );
	}
	/**
	 * Sets the right expression value of this condition.
	 * 
	 * @param rightExpression
	 *            the right expression to set
	 */
	public void setRightExpression( String rightExpression )
	{
		setPropertySilently( JoinCondition.RIGHT_EXPRESSION_MEMBER,
				rightExpression );
	}
	/**
	 * Returns right expression value this condition.
	 * 
	 * @return the right expression value
	 */
	public String getRightExpression( )
	{
		return getStringProperty( JoinCondition.RIGHT_EXPRESSION_MEMBER );
	}
}