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
Element group = svggc.dom.createElement( "g" ); 
			group.setAttribute( "id", "legend" ); 
			svggc.pushParent( group );
		}
		else if ( block instanceof Plot )
		{
			Element group = svggc.dom.createElement( "g" ); 
			group.setAttribute( "id", "plot" ); 
			svggc.pushParent( group );
		}
		else if ( block instanceof LabelBlock )
		{
			Element group = svggc.dom.createElement( "g" ); 
			group.setAttribute( "id", "label" ); 
			svggc.pushParent( group );
		}
		else if ( block instanceof Series )
		{
			Element group = svggc.dom.createElement( "g" ); 
			group.setAttribute( "id", "series" ); 
			svggc.pushParent( group );
		}
		else if ( block instanceof DataPointHints )
		{
			Element group = svggc.dom.createElement( "g" ); 
			group.setAttribute( "id", "dp" + block.hashCode( ) ); 
			svggc.pushParent( group );
		}
	}
	/*