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
((SVGGraphics2D)_g2d).addCSSStyle(".class"+seDT.hashCode(), "visibility", "visible"); 
							}
						}
						catch ( ChartException oosx )
						{
							logger.log( oosx );
							return;
						}
					}
					break;
				case ActionType.HIGHLIGHT :
					
					if ( src.getType( ) == StructureType.SERIES )
					{
						final Series seRT = (Series) src.getSource( );					
						logger.log( ILogger.INFORMATION,
								Messages.getString( "info.toggle.visibility", 
										getLocale() )
										+ seRT );
						String scriptEvent = getJsScriptEvent(tg.getCondition().getValue());
						if (scriptEvent != null){
							
							Series seDT = null;
							try
							{