A follow-up of 675859:

In the IDL I now have:

```
FooCallback = void();

interface I { void f(FooCallback? callback); }
```

However the generated code does not pass nullptr to the C++ implementation if the page has the following in js:
```
I.f(null);
```

I checked the generated bindings and it's creating a Blink callback object even if it's null.