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
}
	
	protected Image createImage( byte[] data )
	{
		return new SVGImage(super.createImage(data), null, data );
	}
	
	public void drawLine(LineRenderEvent lre) throws ChartException {
		groupPrimitive(lre, false);
		super.drawLine(lre);
		ungroupPrimitive(lre, false);
	}
	public void drawOval(OvalRenderEvent ore) throws ChartException {
		groupPrimitive(ore, false);
		super.drawOval(ore);
		ungroupPrimitive(ore, false);
	}
	public void drawPolygon(PolygonRenderEvent pre) throws ChartException {
		groupPrimitive(pre, false);
		super.drawPolygon(pre);
		ungroupPrimitive(pre, false);
	}
	public void drawRectangle(RectangleRenderEvent rre) throws ChartException {
		groupPrimitive(rre, false);
		super.drawRectangle(rre);
		ungroupPrimitive(rre, false);
	}
	public void drawText(TextRenderEvent tre) throws ChartException {
		groupPrimitive(tre, true);
		super.drawText(tre);