JavaScriptCore: More missing static modifiers.
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.