Null safety error for iterating on nullable could be improved

```haxe
@:nullSafety class Main {
static var a:Null>;

static function main() {
for (i in a) {}
}
}
```

```
source/Main.hx:5: characters 3-18 : Null safety: Cannot perform array access on nullable value.
source/Main.hx:5: characters 3-18 : Null safety: Cannot access "length" of a nullable value.
```

The errors are slightly confusing, because the user is not actually perfoming array access or accessing `length` in that line. "Cannot iterate on nullable value" would be better.