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
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import org.eclipse.birt.chart.computation.DataPointHints;
import org.eclipse.birt.chart.device.IDeviceRenderer;
import org.eclipse.birt.chart.device.IUpdateNotifier;
import org.eclipse.birt.chart.device.extension.i18n.Messages;
import org.eclipse.birt.chart.device.plugin.ChartDeviceExtensionPlugin;
import org.eclipse.birt.chart.device.svg.plugin.ChartDeviceSVGPlugin;
import org.eclipse.birt.chart.device.swing.SwingRendererImpl;
import org.eclipse.birt.chart.event.ArcRenderEvent;
import org.eclipse.birt.chart.event.AreaRenderEvent;
import org.eclipse.birt.chart.event.ImageRenderEvent;
import org.eclipse.birt.chart.event.InteractionEvent;
import org.eclipse.birt.chart.event.LineRenderEvent;
import org.eclipse.birt.chart.event.OvalRenderEvent;
import org.eclipse.birt.chart.event.PolygonRenderEvent;
import org.eclipse.birt.chart.event.PrimitiveRenderEvent;
import org.eclipse.birt.chart.event.RectangleRenderEvent;
import org.eclipse.birt.chart.event.StructureChangeEvent;
import org.eclipse.birt.chart.event.StructureSource;
import org.eclipse.birt.chart.event.StructureType;
import org.eclipse.birt.chart.event.TextRenderEvent;
import org.eclipse.birt.chart.event.WrappedStructureSource;
import org.eclipse.birt.chart.exception.ChartException;
import org.eclipse.birt.chart.log.ILogger;
import org.eclipse.birt.chart.log.Logger;
import org.eclipse.birt.chart.model.Chart;
import org.eclipse.birt.chart.model.ChartWithAxes;
import org.eclipse.birt.chart.model.ChartWithoutAxes;