Add empty option for not-required parameter in Widget

For command parameter that is not required but candidate options exist,
DefaultWidgetModel gave no empty option to be chosen from, which force
the user to use the potentially invalid values.

Author: LeonYang5114

src/main/java/org/scijava/widget/DefaultWidgetModel.java

@@ -83,6 +83,8 @@ public DefaultWidgetModel(final Context context, final InputPanel<?, ?> inputPan
 		this.module = module;
 		this.item = item;
 		this.objectPool = objectPool;
+		if (!item.isRequired())
+			this.objectPool.add(0, null);
 		convertedObjects = new WeakHashMap<Object, Object>();
 	}
 
@@ -306,8 +308,10 @@ private Object ensureValidObject(final Object value) {
 
 	/** Ensures the value is on the given list. */
 	private Object ensureValid(final Object value, final List<?> list) {
+		if (value == null)
+			return list.contains(null);
 		for (final Object o : list) {
-			if (o.equals(value)) return value; // value is valid
+			if (value.equals(o)) return value; // value is valid
 			// check if value was converted and cached
 			final Object convertedValue = convertedObjects.get(o);
 			if (convertedValue != null && value.equals(convertedValue)) {