Updating class javafx.beans.binding.When

[Nir Lisker]

Hi,

I was looking at https://bugs.openjdk.java.net/browse/JDK-8089579, which
prompted me to have a look at When. There are a few points I would like to
address:

* StringConditionBuilder#otherwise(ObservableStringValue) does not check
for null as other condition builders do. This results in a deeper NPE
when StringCondition tries to register a listener to the
ObservableStringValue.

* I would like a (re)evaluation on the above bug ticket and thoughts on the
proposal of "on demand evaluation" using a Supplier or a similar method.
The behavior of the intended implementation would be to evaluate 'then' and
'otherwise' whenever their condition is met, and only then.

* The class can benefit from some small refactoring, such as using
Objects.requireNonNull for null checks and some code reuse to reduce the
chance of bugs such as the missing null check of StringConditionBuilder.

* There are a few Javadoc corrections and some clarifications of the
current behavior could be beneficial as well.

I can work on all of the above. How to proceed?

- Nir