resolves #37, jinja if conditional without else returns undefined

[arifb]

See #37 - when the conditional was evaluating to false, jinja returns an undefined object. From http://jinja.pocoo.org/docs/dev/templates/#if-expression

The else part is optional. If not provided, the else block implicitly evaluates into an undefined object

[a-chernykh]

Thanks, @arifb