[cpp] extern class with default args generates null args

Extern classes with default args seem to generate `null()` for unspecified args in the generated C++.

As an example, for the function signature

``` Haxe
public function func(a:Int, b:Int = 1, c:cpp.Float32 = 1.0):Void;
```

and a function call

``` Haxe
e.func(10);
```

Haxe will generate the C++ of

``` c++
e->func((int)10,null(),null());
```

Full example, though the actual C++ will not build:

``` Haxe
package;

extern class ExternClass {

public function func(a:Int, b:Int = 1, c:cpp.Float32 = 1.0):Void;

}

class Main {

public static function main() {
}

public function test(e:ExternClass):Void {

e.func(10);

}

}
```