`undefined` JavaScript value evaluating to truthy in Twing

As I mentioned in another issue, I'm finding that "truthiness" checks in Twing for undefined JavaScript values are evaluating to true instead of the expected false.

For example, if I define a variable in JavaScript like const someVal = undefined, and then pass someVal to a Twig template, then syntax like {% if someVal %}foo{% endif %} or {{ someVal ? 'foo' }} in the Twig file is resulting in 'foo' getting rendered. I believe this is new behavior in 5.1.1.

I'd characterize this as a fairly urgent issue. An undefined value passed in should definitely be interpreted as falsy!