Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/JoinConditionHandle.java
/**
	 * Sets the join type value of this condition.
	 * 
	 * @param type
	 *            the join type to set
	 * @throws SemanticException
	 */
	public void setJoinType( String type ) throws SemanticException
	{
		setProperty( JoinCondition.JOIN_TYPE_MEMBER, type );
	}
	/**
	 * Returns join type value this condition.
	 * 
	 * @return the join type value
	 */
	public String getJoinType( )
	{
		return getStringProperty( JoinCondition.JOIN_TYPE_MEMBER );
	}
	/**
	 * Sets the operator value of this condition.
	 * 
	 * @param operator
	 *            the operator to set
	 * @throws SemanticException
	 */
	public void setOperator( String operator ) throws SemanticException
	{
		setProperty( JoinCondition.JOIN_OPERATOR_MEMBER, operator );