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
* (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.chart.device.IPrimitiveRenderer#enableInteraction(org.eclipse.birt.chart.event.InteractionEvent)
	 */
	public void enableInteraction( InteractionEvent ie ) throws ChartException
	{
		if ( _iun == null )
		{
			logger.log( ILogger.WARNING,
					Messages.getString( "exception.missing.component.interaction", getLocale( ) ) ); 
			return;
		}
		
		Trigger[] triggers = ie.getTriggers( );
		if ( triggers == null )
		{
			return;
		}
		
		 
		
		
		final PrimitiveRenderEvent pre = ie.getHotSpot( );
		Element elm = null;
		if ( pre instanceof PolygonRenderEvent )
		{
			final Location[] loa = ( (PolygonRenderEvent) pre ).getPoints( );
			int[][] pa = getCoordinatesAsInts( loa );
			elm = svggc.createPolygon( pa[0], pa[1], pa[0].length );
		}