[bug] switching on optional field with type option results in totally weird warning

```
type t = {b?: option<int>}
let a: t = Obj.magic()


switch a {
  | {b: None} => ()
  | {b: Some(_)} => ()
}
```

gives warning:

```
[W] Line 5, column 0:

You forgot to handle a possible case here, for example: 
  | {b: None, _}
```

you could even add that particular case:

```
switch a {
  | {b: None, _} => ()
  | {b: Some(_)} => ()
}
```

and you'll still get the exact same warning