default filter is not working with an empty string

[IrakliJani]

default filter is not working with an empty string.
For example: https://jsfiddle.net/q3vL9dkn/

{{ null | default: "default on null"}}
=> "default on null"

{{ "" | default: "default on empty string"}}
=> ""

This works well with Shopify's Liquid (check out last example): https://shopify.github.io/liquid/filters/default/

I have read source code for default filter and Differences With shopify/liquid (about truthy values) but I think we should make an exception for default filter and add an extra check for an empty string.

I can make a PR for it If you think it's a good idea.
Thanks!