Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/SortKeyHandle.java
}
	/**
	 * Returns the sort direction. The possible values are define in
	 * {@link org.eclipse.birt.report.model.api.elements.DesignChoiceConstants}, and they
	 * are:
	 * <ul>
	 * <li>SORT_DIRECTION_ASC
	 * <li>SORT_DIRECTION_DESC
	 * </ul>
	 * 
	 * @return the direction to sort
	 */
	public String getDirection( )
	{
		return getStringProperty( SortKey.DIRECTION_MEMBER );
	}
	/**
	 * Sets the sort direction. The allowed values are define in
	 * {@link org.eclipse.birt.report.model.api.elements.DesignChoiceConstants}, and they
	 * are:
	 * <ul>
	 * <li>SORT_DIRECTION_ASC
	 * <li>SORT_DIRECTION_DESC
	 * </ul>
	 * 
	 * @param direction
	 *            the direction to set
	 * @throws SemanticException
	 *             if the direction is not in choice list.
	 */