Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
public void removeItem( ElementPropertyDefn prop, int posn )
			throws SemanticException
	{
		assert prop != null;
		
		assertExtendedElement( module, element, prop );
		checkListProperty( prop );
		List list = element.getListProperty( module, prop.getName( ) );
		if ( list == null )
			throw new PropertyValueException( element, prop.getName( ), null,
					PropertyValueException.DESIGN_EXCEPTION_ITEM_NOT_FOUND );
		if ( posn < 0 || posn >= list.size( ) )
			throw new IndexOutOfBoundsException(
					"Posn: " + posn + ", List Size: " + list.size( ) ); 
		Object obj = list.get( posn );
		doRemoveItem( prop, posn, obj );
	}
	/**
	 * Removes a value item from a simple value list.
	 * <ul>
	 * <li>The item must exist in the current effective value for the list.
	 * This means the list must be set on this element or a ancestor element.
	 * </li>
	 * <li>If the property is set on this element, then the item is simply
	 * removed.</li>
	 * <li>If the property is set on an ancestor element, then the inherited
	 * list is first <strong>copied </strong> into this element. Then, the copy
	 * of the target item is removed from the copy of the list.</li>
	 * </ul>
	 *