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
}
	/**
	 * The SVG version is "-W3CDTD SVG 1.0EN".
	 */
	static private final String SVG_VERSION = "-W3CDTD SVG 1.0EN"; 
	/**
	 * The SVG DTD is
	 * "http:www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd".
	 */
	static private final String SVG_DTD = "http:www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"; 
	
	/**
	 * The SVG namespace http:www.w3.org/2000/svg
	 */
	static private final String XMLNS = "http:www.w3.org/2000/svg"; 
	/**
	 * The  xmlns:xlink is
	 * "http:www.w3.org/1999/xlink".
	 */
	static private final String XMLNSXINK = "http:www.w3.org/1999/xlink"; 
	
	/**
	 * 
	 */
	private Object oOutputIdentifier = null;
	/**
	 * Document object that represents the SVG
	 */
	protected Document dom;