Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
stack.commit( );
	}
	/**
	 * Adds an item to a structure list.
	 * <ul>
	 * <li>If the property is currently unset anywhere up the inheritance
	 * hierarchy, then a new list is created on this element, and the list
	 * contains the only the new item.</li>
	 * <li>If the property is currently set on this element, then the item is
	 * added to the existing list.</li>
	 * <li>If the list is not set on this element, but is set by an ancestor
	 * element, then the list is <strong>copied </strong> onto this element, and
	 * the new element is then appended to the copy.</li>
	 * </ul>
	 * 
	 * @param ref
	 *            reference to the list into which to add the structure
	 * @param item
	 *            the structure to add to the list
	 * @throws SemanticException
	 *             if the item to add is invalid.
	 */
	public void addItem( MemberRef ref, IStructure item )
			throws SemanticException
	{
		assert ref != null;
		if ( item == null )
			return;
		PropertyDefn propDefn = ref.getPropDefn( );
		assert propDefn != null;