[cpp] Unable to use Class references tagged with `@:nativeGen`

I was testing with the capabilities of `@:nativeGen` when I noticed that apparently if you want to reference a class for various reasons (for example, `Std.isOfType()`) it will return false regardless if it's a correct input.

Looking further, it seems to be because the Haxe compiler automatically sets `null` to any kind of reference of the core class.

Example Source:
```haxe
package;

@:nativeGen
class MyClass {
public function new(?arg:String)
{
trace(arg);
}
}
@:nativeGen
class MyOtherClass extends MyClass
{
public function new()
{
super("Overrided");
}
}

@:nativeGen
class Main
{
static function main() {

var int = Std.parseInt("234");

var thing = MyClass;

if (MyOtherClass is MyClass)
trace(int);
new MyClass("My Class Test");
}
}
```

Output:

```cpp
// Generated by Haxe 4.3.6
#include

#ifndef INCLUDED_6dd33a6ac0a7028f
#define INCLUDED_6dd33a6ac0a7028f
#include "Main.h"
#endif
#ifndef INCLUDED_f94a69a5dd97ef77
#define INCLUDED_f94a69a5dd97ef77
#include "MyClass.h"
#endif
#ifndef INCLUDED_1dc702ddb70657b6
#define INCLUDED_1dc702ddb70657b6
#include "MyOtherClass.h"
#endif
#ifndef INCLUDED_58b6f7d5e7ae9899
#define INCLUDED_58b6f7d5e7ae9899
#include "Std.h"
#endif
#ifndef INCLUDED_haxe_Log
#include
#endif

HX_LOCAL_STACK_FRAME(_hx_pos_e47a9afac0942eb9_22_main,"Main","main",0xed0e206e,"Main.main","Main.hx",22,0x087e5c05)

void Main::main(){
HX_STACKFRAME(&_hx_pos_e47a9afac0942eb9_22_main)
HXLINE( 24) ::Dynamic _hx_int = ::Std::parseInt(HX_("234",53,1d,26,00));
HXLINE( 26) ::Dynamic thing = null();
HXLINE( 28) if (::Std::isOfType(null(),null())) {
HXLINE( 29) ::haxe::Log_obj::trace(_hx_int,::hx::SourceInfo(HX_("Main.hx",05,5c,7e,08),29,HX_("Main",59,64,2f,33),HX_("main",39,38,56,48)));
}
HXLINE( 30) ::MyClass(HX_("My Class Test",8e,3d,75,75));
}

```

the expected output should be that the sequence "234" is outputted, but not in this case.

Start by reproducing the issue with the Haxe source example and inspect the generated C++ around `thing` and the `MyOtherClass is MyClass` check. Trace why these class references become `null`; done means the generated program outputs `234` while preserving the shown class construction behavior.

Written by the indexing model from the issue text.