[cpp] Null<UInt> null check fails

Haxe code:

```haxe
class Test {
static function test (color : Null = null) {
trace (color != null ? 'not null' : 'null');
}
static function main() {
test ();
}
}
```

Haxe 3.4.7 output:
js: null
cpp: null

Generated cpp code:
```cpp
HXDLIN( 4) ::Dynamic _hx_tmp;
HXDLIN( 4) if (hx::IsNotNull( color )) {
HXDLIN( 4) _hx_tmp = HX_("not null",b4,90,c2,b2);
} else {
HXDLIN( 4) _hx_tmp = HX_("null",87,9e,0e,49);
}
```

Haxe 4.1.1 output:
js: null
cpp: **not null**

Generated cpp code:
```cpp
HXDLIN( 4) ::Dynamic b = null();
HXDLIN( 4) if (::hx::IsNotEq( ( (int)(color) ),b )) {
HXDLIN( 4) _hx_tmp = HX_("not null",b4,90,c2,b2);
}
else {
HXDLIN( 4) _hx_tmp = HX_("null",87,9e,0e,49);
}
```