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
for (int x = 0; x< scriptCodeList.size(); x++){
				String code = (String)scriptCodeList.get(x);
				((SVGGraphics2D)_g2d).addScript(code);				
			}
		}
		if (this.scriptRefList != null){
			for (int x = 0; x< scriptRefList.size(); x++){
				String ref = (String)scriptRefList.get(x);
				((SVGGraphics2D)_g2d).addScriptRef(ref);				
			}
		}
	}
	/**
	 * 
	 * @param os
	 * @throws RenderingException
	 */
	public final void after( ) throws ChartException
	{
		super.after( );
		addScripts();
		((SVGGraphics2D)_g2d).flush();		
		
		
		dom.getDocumentElement().appendChild(hotspotLayer);
		
		if ( oOutputIdentifier instanceof OutputStream ) 
		{
			try
			{