Fix EZP-23553: Checking for existence of legacy templates does not work

JIRA issue: https://jira.ez.no/browse/EZP-23553

When trying to do something like this:

```twig
{% include [
        'NetgenSiteBundle:parts:' ~ template ~ '.html.twig',
        'design:parts/' ~ template ~ '.tpl',
        'design:parts/default.tpl'
    ] ignore missing
    with {
        'location': location,
        'node': location
    }
%}
```

And first two templates do not exist, but the third one does, nothing will be rendered, while the third template should be rendered.

What happens is that legacy rendering engine does not recognize a missing legacy template properly and instead returns an empty string to be rendered.