JavaScriptCore: ConvertPtr should allow JavaScript null to be a valid value.

It is common in C to accept NULL to functions for pointer parameters.
extern void DoSomething(struct Foo* foo);
...
DoSomething(NULL);

Thus, JavaScript null should be allowed:
module.DoSomething(null);

But the current ConvertPtr definition accepts only objects. This modifies it to allow null.
Javascript: support null pointers.

We allow to set pointer types using JS null.
JavascriptCore: Bugfix for null-pointer support.

This solution must be considered as a preliminary workaround.