Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/FilterConditionHandle.java
case OperatorUtil.OPERATOR_LEVEL_ZERO :
					setValue2( null );
					setValue1( null );
					break;
				case OperatorUtil.OPERATOR_LEVEL_NOT_EXIST :
					break;
			}
		}
		catch ( SemanticException e )
		{
			stack.rollback( );
			throw e;
		}
		stack.commit( );
	}
	/**
	 * Returns the value 1 expression of this filter condition.
	 * 
	 * @return the value 1 expression of this filter condition
	 */
	public String getValue1( )
	{
		return getStringProperty( FilterCondition.VALUE1_MEMBER );
	}
	/**
	 * Sets the value 1 expression of this filter condition.
	 * 
	 * @param value1Expr
	 *            the value 1 expression to set
	 */