Fix NPE in VmTyped.hashCode when property is null

[jurajmaj]

Fixes #1167

Null properties in VmTyped previously caused a NPE when accessed through hashCode().
This fix adds null checks so that containsKey and other map operations do not crash.

[jurajmaj]

Hi @HT154, this PR is related to the issue we discussed #1167. I just wanted to check if there’s any feedback.
Thank you for your time!

[HT154]

Hi @jurajmaj, thanks for the contribution! Most of the team is away until December, so it'll take us a little more time to review this fully. Thanks for your patience!

That said, my gut feeling here is that isn't quite the right way to resolve this. The cached property value being null here is a symptom of a Pkl stack overflow (circular data reference), so that should be the error communicated to the user. Interestingly, the exception's Pkl stack trace has already been rendered accordingly!