Revert "Add empty option for not-required parameter in Widget"

This reverts commit 45ab6cf.

This will require a revision of how the ObjectPool is handled in
widgets, as it currently breaks the javadoc contract (in that if a null
value is added to the ObjectPool, the item value is forced to be null).

Author: hinerm

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

@@ -87,8 +87,6 @@ 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>();
 
 		if (item.getValue(module) == null) {
@@ -317,10 +315,8 @@ 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 (value.equals(o)) return value; // value is valid
+			if (o.equals(value)) return value; // value is valid
 			// check if value was converted and cached
 			final Object convertedValue = convertedObjects.get(o);
 			if (convertedValue != null && value.equals(convertedValue)) {