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
case TriggerCondition.ONCLICK :
			return"onclick"; 
		case TriggerCondition.ONDBLCLICK :
			return"onclick"; 
		case TriggerCondition.ONMOUSEDOWN :
			return"onmousedown"; 
		case TriggerCondition.ONMOUSEUP :
			return"onmouseup"; 
		case TriggerCondition.ONMOUSEOVER :
			return"onmouseover"; 
		case TriggerCondition.ONMOUSEMOVE :
			return"onmousemove"; 
		case TriggerCondition.ONMOUSEOUT :
			return"onmouseout"; 
		case TriggerCondition.ONFOCUS :
			return"onfocusin"; 
		case TriggerCondition.ONBLUR :
			return"onfocusout"; 
		case TriggerCondition.ONKEYDOWN :
			return"onkeydown"; 
		case TriggerCondition.ONKEYPRESS :
			return"onkeypress"; 
		case TriggerCondition.ONKEYUP :
			return"onkeyup"; 
		case TriggerCondition.ONLOAD :
			return"onload"; 
		}
		return null;
		
	}