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 < triggers.length; x++) {
				Trigger tg = triggers[x];
				
				final StructureSource src = (StructureSource) ie.getSource( );
				
				switch (tg.getAction().getType().getValue()) {
				case ActionType.SHOW_TOOLTIP:
					Element title = svggc.dom.createElement("title"); 
					title.appendChild(svggc.dom
							.createTextNode(((TooltipValue) tg.getAction()
									.getValue()).getText()));
					elm.appendChild(title);
					elm.setAttribute("onmouseout", "TM.remove()"); 
					elm.setAttribute("onmouseover", "TM.show(evt)"); 		
					break;
				case ActionType.URL_REDIRECT:
					URLValue urlValue = ((URLValue) tg.getAction().getValue());
					Element aLink = ((SVGGraphics2D)_g2d).createElement("a"); 
					aLink.setAttribute("xlink:href", urlValue.getBaseUrl()); 
					if (urlValue.getTarget() != null)
						aLink.setAttribute("target", urlValue.getTarget());  
					aLink.appendChild(elm);
					elm = aLink;
					break;
				case ActionType.TOGGLE_VISIBILITY :
					
					if ( src.getType( ) == StructureType.SERIES )
					{
						final Series seRT = (Series) src.getSource( );					
						logger.log( ILogger.INFORMATION,