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
hotspotLayer = createHotspotLayer(dom);
				super.setProperty( IDeviceRenderer.GRAPHICS_CONTEXT, svggc );
			}
			catch ( Exception e )
			{
				e.printStackTrace( );
			}
		}
		else if ( sProperty.equals( IDeviceRenderer.FILE_IDENTIFIER ) )
		{
			oOutputIdentifier = oValue;
		}
		else if ( sProperty.equals( ISVGConstants.JAVASCRIPT_CODE_LIST ) )
		{
			scriptCodeList = (List)oValue;
		}
		else if ( sProperty.equals( ISVGConstants.JAVASCRIPT_URL_REF_LIST ) )
		{
			scriptRefList = (List)oValue;
		}
	}
	
	protected Element createHotspotLayer(Document dom){
		Element hotspot = dom.createElement("g"); 
		hotspot.setAttribute( "style", "fill-opacity:0.01;fill:#FFFFFF;" ); 
		return hotspot;		
	}
	
	protected void addScripts(){
		if (this.scriptCodeList != null){