Bug 124777 Set blank tooltip in chart title will cause exception when preview.
Description: Set blank tooltip in chart title will cause exception when preview. Steps to reproduce: 1. New a simple chart 2. Add title interactivity: mouse over/show tooltip, keep tooltip text blank and click Add button. 3. Preview the chart in viewer and move mouse over the chart title. Expected result: Nothing happen. Actual result: Unexpected exception dialog poped up.
chart/org.eclipse.birt.chart.device.svg/src/org/eclipse/birt/chart/device/svg/SVGRendererImpl.java
return svgDocument;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.chart.device.IStructureDefinitionListener#changeStructure(org.eclipse.birt.chart.event.StructureChangeEvent)
	 */
	public void changeStructure( StructureChangeEvent scev )
	{
	}
	protected void removeGroupStructure( Object block )
	{
		if ( ( block instanceof TitleBlock )
				|| ( block instanceof Legend )
				|| ( block instanceof Plot )
				|| ( block instanceof LabelBlock )
				|| ( block instanceof Series )
				|| ( block instanceof DataPointHints ) )
			svggc.popParent( );
	}
	protected void addGroupStructure( Object block )
	{
		if ( block instanceof TitleBlock )
		{
			Element group = svggc.dom.createElement( "g" ); 
			group.setAttribute( "id", "title" ); 
			svggc.pushParent( group );
		}
		else if ( block instanceof Legend )
		{