Bug 164247 [DataBinding] NPE from ObservableMapLabelProvider when value is null
When a null value is to be returned from getColumnText(...) a NPE is thrown. It's trying to do a toString() on the null value.
bundles/org.eclipse.jface.databinding/src/org/eclipse/jface/databinding/viewers/ObservableMapLabelProvider.java
implements ILabelProvider, ITableLabelProvider {
	private final IObservableMap[] attributeMaps;
	private IMapChangeListener mapChangeListener = new IMapChangeListener() {
		public void handleMapChange(IObservableMap source, MapDiff diff) {
			Set affectedElements = diff.getChangedKeys();
			LabelProviderChangedEvent event = new LabelProviderChangedEvent(
					ObservableMapLabelProvider.this, affectedElements
							.toArray());
			fireLabelProviderChanged(event);
		}
	};
	/**
	 * @param attributeMap
	 */
	public ObservableMapLabelProvider(IObservableMap attributeMap) {
		this(new IObservableMap[] { attributeMap });
	}
	/**
	 * @param attributeMaps