[interop] foreign null objects not handled correctly as falsy values

[fniephaus]

Repro:

$ polyglot --jvm --shell
GraalVM MultiLanguage Shell 19.3.0
  Python version 3.7.4
  Ruby version 2.6.2
ruby> if nil; puts "No!!" end
nil
ruby> if Polyglot.eval('python', 'None'); puts "No!!!" end
No!!! # not expected!
nil
ruby> Polyglot.eval('python', 'None').nil?
true

[eregon]

Should isNull() imply "false when converted to boolean"?

I agree for None, but unsure, e.g., for a pointer object which has an address of 0 and also implements isNull() for some reason, such as NFI NativePointer.

[fniephaus]

Intuitively, I would have expected a null pointer to behave as false in conditions. It would also return true for nil?, right?

[eregon]

Right, we handle foreign.nil? by sending isNull().

Do you know if any other language (except Sulong) does this?
Maybe we should add this in the JavaDoc of isNull()? cc @boris-spas @chumer

[fniephaus]

In GraalSqueak, we map ForeignObject>>isNil to isNull(). However, it's more common to use ifNil:ifNotNil:. That's also why the Squeak parser optimizes these cases by replacing ifNil: with a nil identity check, which is unfortunate because nil != Ruby's nil or Python's None.