Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
stack.execute( record );
		if ( oldItem.isReferencable( ) )
			adjustReferenceClients( (ReferencableStructure) oldItem );
		stack.commit( );
	}
	/**
	 * Removes all contents of the list. This is different from simply clearing
	 * the property. Removing all the contents leaves the property set to an
	 * empty list.
	 * 
	 * @param ref
	 *            reference to the list to clear
	 * @throws SemanticException
	 *             if the property is not a structure list property
	 */
	public void removeAllItems( MemberRef ref ) throws SemanticException
	{
		checkListMemberRef( ref );
		PropertyDefn propDefn = ref.getPropDefn( );
		assert propDefn != null;
		assertExtendedElement( module, element, propDefn );
		if ( ref.refType == MemberRef.PROPERTY )
			setProperty( ref.getPropDefn( ), null );
		else
			setMember( ref, null );
	}
	/**
	 * Moves an item within a list from one position to a new position.
	 * <ul>
	 * <li>The element must exist in the current effective value for the list.