Keep undefined/falsy value from model

[MasGaNo]

On View class, when we provide to template all variable from Model, templateVariables, ... the falsy/undefined values can be removed from Model.

It's because of this code: https://github.com/jashkenas/underscore/blob/master/underscore.js#L1082

And because of that, we need to ensure that variable exists in the template because it throws an error if the variable doesn't exist.