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
* Helper function that will determine if the source object is a series component of the chart.
	 * 
	 * @param src StructureSource that is stored in the primitive render event.
	 * @return null if the source object is not a series otherwise it returns the StructureSource.
	 */
	private StructureSource isSeries(StructureSource src){
		if (src instanceof WrappedStructureSource){
			WrappedStructureSource wss = (WrappedStructureSource)src;
			while (wss != null){
				if (wss.getType() == StructureType.SERIES) {
					return wss;
				}
				if (wss.getParent().getType() == StructureType.SERIES){
					return wss.getParent();
				}
				if (wss.getParent() instanceof WrappedStructureSource)
					wss = (WrappedStructureSource)wss.getParent();
				else
					wss = null;
			}
		}
		else if (src.getType() == StructureType.SERIES) return src;
		return null;
	}
	/**
	 * Groups the svg drawing instructions that represents this primitive events.  Each group is 
	 * assigned an id that identifies the source object of the primitive event
	 * 
	 * @param pre primitive render event
	 * @param drawText TODO