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
else if ( pre instanceof OvalRenderEvent )
		{
			final Bounds boEllipse = ( (OvalRenderEvent) pre ).getBounds( );
			elm = svggc.createOval( boEllipse.getLeft( ),
					boEllipse.getTop( ),
					boEllipse.getWidth( ),
					boEllipse.getHeight( ) );
		}
		else if ( pre instanceof ArcRenderEvent )
		{
			final ArcRenderEvent are = (ArcRenderEvent) pre;
			
			if ( are.getInnerRadius( ) >= 0
					&& are.getOuterRadius( ) > 0
					&& are.getInnerRadius( ) < are.getOuterRadius( ) )
			{
				Shape outerArc = new Arc2D.Double( are.getTopLeft( ).getX( )
						+ ( are.getWidth( ) - 2 * are.getOuterRadius( ) )
						/ 2,
						are.getTopLeft( ).getY( )
								+ ( are.getHeight( ) - 2 * are.getOuterRadius( ) )
								/ 2,
						2 * are.getOuterRadius( ),
						2 * are.getOuterRadius( ),
						are.getStartAngle( ),
						are.getAngleExtent( ),
						Arc2D.PIE );
				Shape innerArc = new Arc2D.Double( are.getTopLeft( ).getX( )
						+ ( are.getWidth( ) - 2 * are.getInnerRadius( ) )
						/ 2,