[nullsafety] indirect null checks

It seems like the following should compile:

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

static function main() {
var hasFoo = foo != null;
if (hasFoo) {
foo.charAt(0);
}
}
}
```

```
source/Main.hx:8: characters 4-14 : Null safety: Cannot access "charAt" of a nullable value.
```