Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
* The top level element property referenced by a member reference can be a
	 * list property or a structure property.
	 * <p>
	 * <li>If references a list property, the method will check to see if the
	 * current element has the local list value, if it has, the method returns,
	 * otherwise, a copy of the list value inherited from container or parent
	 * will be set locally on the element itself.
	 * <li>If references a structure property, the method will check to see if
	 * the current element has the local structure value, if it has, the method
	 * returns, otherwise, a copy of the structure value inherited from
	 * container or parent will be set locally on the element itself.
	 * <p>
	 * This method is supposed to be used when we need to change the value of a
	 * composite property( a list property or a structure property ). These kind
	 * of property is inherited as a whole, so when the value changed from a
	 * child element. This method will be called to ensure that a local copy
	 * will be made, so change to the child won't affect the original value in
	 * the parent.
	 * 
	 * @param ref
	 *            a reference to a list property or member.
	 */
	void makeLocalCompositeValue( MemberRef ref )
	{
		assert ref != null;
		ElementPropertyDefn propDefn = ref.getPropDefn( );
		if ( ref.getPropDefn( ).isList( ) )
		{
			
			ArrayList list = (ArrayList) element.getLocalProperty( module,