Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
*            the new property value.
	 * @throws ExtendedElementException
	 *             if the extension property is invalid
	 * @throws PropertyValueException
	 *             if the element is a template element and users try to set the
	 *             value of template definition to "null" or a non-exsiting
	 *             element
	 */
	private void doSetProperty( ElementPropertyDefn prop, Object value )
			throws ExtendedElementException, PropertyValueException
	{
		
		
		
		Object oldValue = element.getLocalProperty( module, prop );
		if ( oldValue == null && value == null )
			return;
		if ( oldValue != null && value != null && oldValue.equals( value ) )
			return;
		
		if ( element instanceof ExtendedItem )
		{
			ExtendedItem extendedItem = ( (ExtendedItem) element );
			if ( extendedItem.isExtensionModelProperty( prop.getName( ) )
					|| extendedItem.isExtensionXMLProperty( prop.getName( ) ) )
			{
				IReportItem extElement = extendedItem.getExtendedElement( );
				assert extElement != null;
				extElement.checkProperty( prop.getName( ), value );
				extElement.setProperty( prop.getName( ), value );