Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/FilterConditionHandle.java
* <li><code>FILTER_OPERATOR_TRUE</code>
	 * <li><code>FILTER_OPERATOR_FALSE</code>
	 * <li><code>FILTER_OPERATOR_LIKE</code>
	 * <li><code>FILTER_OPERATOR_TOP_N</code>
	 * <li><code>FILTER_OPERATOR_BOTTOM_N</code>
	 * <li><code>FILTER_OPERATOR_TOP_PERCENT</code>
	 * <li><code>FILTER_OPERATOR_BOTTOM_PERCENT</code>
	 * <li><code>FILTER_OPERATOR_ANY</code>
	 * </ul>
	 * 
	 * @param operator
	 *            the operator to set
	 * @throws SemanticException
	 *             if operator is not in the choice list.
	 */
	public void setOperator( String operator ) throws SemanticException
	{
		ActivityStack stack = getModule( ).getActivityStack( );
		stack.startTrans( );
		try
		{
			setProperty( FilterCondition.OPERATOR_MEMBER, operator );
			int level = OperatorUtil.computeFilterOperatorLevel( operator );
			switch ( level )
			{
				case OperatorUtil.OPERATOR_LEVEL_ONE :
					setValue2( null );
					break;
				case OperatorUtil.OPERATOR_LEVEL_TWO :
					break;