Consider null as defined #1

In order to create components with optional properties, we need some way of specifying that a reactive property intentionally is defined to be no value. For example, the optional properties for color and size in configuring a scatter plot like this:

Previously, with ModelJS, optional values that have no value were specified using Model.None, a special object for this purpose inspired by Scala's Option Type. However, after reading this great article on null vs. undefined, I realized that null is actually meant for this purpose, and that's why it is distinct from undefined.

Therefore, it would be a Good Idea to consider null as a defined value within reactive-function. This would mean that if an input to a reactive function has been assigned null, then that reactive function will be invoked.