Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/FilterConditionHandle.java
/**
 * Represents one filter in the filter list of List, Table or their Groups.
 * <p>
 * This is a managed object, meaning that all changes should be made though the
 * command layer so that they can be undone and redone. Each filter condition
 * has the following properties:
 * 
 * <p>
 * <dl>
 * <dt><strong>Column </strong></dt>
 * <dd>a filter condition has a required column.</dd>
 * 
 * <dt><strong>Operator </strong></dt>
 * <dd>a filter condition has a required operator to compute.</dd>
 * 
 * <dt><strong>Filter Expression </strong></dt>
 * <dd>a filter condition has a required filter expression to test. Can be a
 * column or a complete boolean expression.</dd>
 * 
 * <dt><strong>Value 1 Expression </strong></dt>
 * <dd>a filter condition has an optional value 1 expression of the comparison
 * value for all but unary operators.</dd>
 * 
 * <dt><strong>Value 2 Expression </strong></dt>
 * <dd>a filter condition has an optional value 2 expression of the second
 * comparison value for trinary operators(between, not between).</dd>
 * </dl>
 * 
 */
public class FilterConditionHandle extends StructureHandle