Set null instead of undefined in V8Objects for null values in Java Map

[GrahamBorland]

This changes V8ObjectUtils.toV8Object() so that it sets null instead of undefined when it encounters a null value in the Java Map.

I appreciate that it may not always be clear which is better. I did wonder about adding a boolean flag to the public interface, or a parallel toV8ObjectUseNull() instead, but I wanted to solicit feedback first.

My opinion: if a Java Map does contain an entry with a null value, then it got there because someone explicitly put it there, so the semantics of the JS null fit better than undefined. It's not like someone just forgot to initialise a field or variable, which is what undefined really means in JS.

In any event, this change does fix a specific bug I encountered where a third-party JS library was handling null and undefined differently.