Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
PropertyDefn memberDefn = (PropertyDefn) iter.next( );
			if ( !ReferencableStructure.LIB_REFERENCE_MEMBER.equals( memberDefn
					.getName( ) ) )
				item.setProperty( memberDefn, memberDefn.validateValue( module,
						item.getLocalProperty( module, memberDefn ) ) );
		}
		if ( item instanceof Structure )
		{
			List errorList = ( (Structure) item ).validate( module, element );
			if ( errorList.size( ) > 0 )
			{
				throw (SemanticException) errorList.get( 0 );
			}
		}
	}
	/**
	 * Adds an item to a property 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 prop
	 *            the property definition whose type is list