Twig implementation does not print int(0)

Problem/Motivation

Drupal's Twig implementation does not print anything for the following:
{% set var = 0 %}{{ var }}

This should print '0'.

Proposed resolution

Track down the bug and squash it. @Fabianx said that there is a check for (!$var) somewhere that should be refactored, and possibly we can check is_numeric().

Remaining tasks

TBD

User interface changes

n/a

API changes

n/a

Related Issues

#1757550: [meta] Convert core theme functions and templates to Twig