Using certain filters on an undefined variable will throw an error

[robinbijlani]

Thank you for the great package! 🙇‍♀

I did run into a situation which I think may be a bug:

If you use any of the following filters on an undefined variable, they will throw an error:

• capitalize
• downcase
• size
• remove
• first
• last

Example: https://jsfiddle.net/wxekqcnp/3/

I would not expect this to happen because with the default options, an undefined variable should evaluate to an empty string, and these filters all work fine on an empty string:
Example: https://jsfiddle.net/wxekqcnp/2/

Do I misunderstand the intended behavior, or is this a bug?