`is iterable` test on `null` or `undefined` values results in incorrect behavior

The is iterable test doesn't always give correct results for values that are null or undefined.

null

In Twing, the following code that defines x as null via a set block doesn't error out (it prints "null is not iterable"):

{% set x %}
  {{ null }}
{% endset %}
{% if x is iterable %}null is iterable{% else %}{{ x }}null is not iterable{% endif %}

whereas the following code that defines x as null via a set "one-liner" errors out with "object null is not iterable":

{% set x = null %}
{% if x is iterable %}null is iterable{% else %}null is not iterable{% endif %}

Passing in x: null from the JavaScript results in the same error.

In TwigPHP, there is no error when checking if null is iterable (it simply resolves to false). So the set-block behavior seems to be correct, and I think that the error that results from the set one-liner or from passing in a null value from the JavaScript is a bug.

undefined (and missing / not-defined)

Relatedly (though less important), checking if an undefined value is iterable throws the "wrong" error. In TwigPHP, you get a helpful "does not exist" runtime error. In Twing, you get a JavaScript "undefined is not iterable (cannot read property Symbol(Symbol.iterator))" error, which is less helpful.

Relatedly, checking if an undefined value is iterable in Twing throws a JavaScript "cannot read property Symbol(Symbol.iterator)" error, which isn't reflective of what happens in TwigPHP for values that haven't been defined. The correct behavior is:

• if strict_variables is enabled, throw a "does not exist" runtime error;
• if strict_variables is disabled (the default), treat the undefined case the same as the null case.