Exhaustiveness checks thrown off by enum abstract with only null value

Weird edge case incoming:

```haxe
import haxe.ds.Option;

class Test {
static function main() {
switch Some(Working) {
case Some(Working | Somehow): // fine
case None:
}

switch Some(Broken) {
case Some(Broken): // [ERROR] Unmatched patterns: Some(_)
case None:
}
}
}

enum abstract Working(Null) {
var Working = null;
var Somehow = 123;
}

enum abstract Broken(Null) {
var Broken = null;
}
```