[cpp] unnecessary Dynamics with optional arguments with default values

This Haxe code:

```haxe
class Main {
public static function main() {
foobar();
foobar2();
}

static function foobar(def:Int = 0) {}
static function foobar2(?def:Int = 0) {}
}
```

Generates the following C++ code:

```cpp
void Main_obj::main(){
HX_STACKFRAME(&_hx_pos_e47a9afac0942eb9_2_main)
HXLINE( 3) ::Main_obj::foobar(null());
HXLINE( 4) ::Main_obj::foobar2(null());
}

STATIC_HX_DEFINE_DYNAMIC_FUNC0(Main_obj,main,(void))

void Main_obj::foobar(hx::Null< int > __o_def){
int def = __o_def.Default(0);
HX_STACKFRAME(&_hx_pos_e47a9afac0942eb9_7_foobar)
}

STATIC_HX_DEFINE_DYNAMIC_FUNC1(Main_obj,foobar,(void))

void Main_obj::foobar2( ::Dynamic __o_def){
::Dynamic def = __o_def.Default(0);
HX_STACKFRAME(&_hx_pos_e47a9afac0942eb9_8_foobar2)
}

STATIC_HX_DEFINE_DYNAMIC_FUNC1(Main_obj,foobar2,(void))
```

Note how `foobar`'s argument is properly typed as `hx::Null`, whereas in `foobar2` it is `Dynamic` (and presumably slower as a result). According to @nadako, it should be possible to generate `foobar2`'s argument as `hx::Null` too in this case.