incorrect union type exhaustive match warning with ClassTag

## Compiler version
3.0.0-RC1

## Minimized code

```Scala
import scala.reflect.ClassTag

type A
given ClassTag[A] = ???
var a: A|Null = null
a match { //WARNING: match may not be exhaustive. It would fail on pattern case: _: A
  case null =>
  case a: A =>
}
```

