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
private static ILogger logger = Logger.getLogger( "org.eclipse.birt.chart.device.svg/trace" ); 
	/**
	 * 
	 */
	private IUpdateNotifier _iun = null;
	
	private Map componentPrimitives = new Hashtable();
	private List scripts = new Vector();
	protected List scriptRefList = null;
	protected List scriptCodeList = null;
	
	/**
	 * 
	 */
	public SVGRendererImpl( )
	{
		if ( System.getProperty( "STANDALONE" ) == null ) 
		{
			final PluginSettings ps = PluginSettings.instance( );
			try
			{
				_ids = ps.getDisplayServer( "ds.SVG" ); 
			}
			catch ( ChartException pex )
			{
				logger.log( pex );
			}
		}
		else
			_ids = new SVGDisplayServer( );