[On hold] Improve undefinedidentifier processing

[fgalan]

Intead of raise a UndefinedIdentifier error when a unknown identifier is found, it is replaced by null in evaluation without raising error.

The PR is still [WIP] as test has to reviwed.

Should this change be proposed into the mainstream repository (https://github.com/mozilla/jexl-rs)?

[fgalan]

After careful examination, it is some grade of support of undefined identifiers in the upgraeam jexl-rs code. From test_undefined_identifier_truthy_ops test case.

Thus, things like this are not working:

        test("UNDEFINED", false, value!(null));
        test("UNDEFINED == 'string'", false, value!(null));
        test("'string' == UNDEFINED", false, value!(null));
        test("DEFINED ? UNDEFINED : 'WRONG'", false, value!(null));

which preclude expressions like (A==null)?'is null'|A.

But following are working:

        test("UNDEFINED ? 'WRONG' : 'RIGHT'", true, value!("RIGHT"));
        test("UNDEFINED || 'RIGHT'", true, value!("RIGHT"));
        test("'RIGHT' || UNDEFINED", true, value!("RIGHT"));

        test(
            "(UNDEFINED && UNDEFINED == 'string') || (DEFINED && DEFINED == 'string')",
            true,
            value!(true),
        );

So instead of (A==null)?'is null'|A we can use just A||'is null'.

We would leave this PR opened by the moment, but it seems we don't need this fix at the end.