Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
* contains the only the new item.</li>
	 * <li>If the property is currently set on this element, then the item is
	 * inserted into 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 inserted into the copy.</li>
	 * </ul>
	 * 
	 * @param ref
	 *            reference to the list into which to insert the new item
	 * @param item
	 *            the item to insert
	 * @param posn
	 *            the position at which to insert the item
	 * @throws SemanticException
	 *             if the item to add is invalid.
	 * @throws IndexOutOfBoundsException
	 *             if the given posn is out of range
	 *             <code>(index &lt; 0 || index &gt; list.size())</code>.
	 */
	public void insertItem( MemberRef ref, IStructure item, int posn )
			throws SemanticException
	{
		assert ref != null;
		if ( item == null )
			return;
		PropertyDefn propDefn = ref.getPropDefn( );
		assert propDefn != null;
		assertExtendedElement( module, element, propDefn );
		checkListMemberRef( ref );