DefaultWidgetModel: guard against null conversions

When the conversion fails, it returns null. In that case, we should
treat the converted value as identical to the original value.

Author: ctrueden

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

@@ -156,6 +156,7 @@ public void setValue(final Object value) {
 
 		// Pass the value through the convertService
 		convertedInput = convertService.convert(value, item.getType());
+		if (convertedInput == null) convertedInput = value;
 
 		// If we get a different (converted) value back, cache it weakly.
 		if (convertedInput != value) {