Fix list truthiness

- [ ] Fix truthiness of list
- [x] Test truthiness of non-empty list
- [x] Test truthiness of empty list
- [ ] [Sign CLA](http://codecombat.com/cla)

This PR will be ready to merge when all tasks are checked (and it passes review).
I'll clean up the commits before giving :+1: on my end.

Closes codecombat/codecombat#1958