Default Support for Include Params

I'd like to implement a coalesce-like workflow for my includes, like I can do in javascript with chaining multiple a || b together, such as var = include.var || page.var || site.var;. It'll reduce repetition in the codebase. The default operator seems to be erroring out when testing against a missing include param. Therfore, instead of:

{% assign odd = include.odd | default:  false %}

I have to do:

{% assign odd = false %}{% if include.odd %}{% assign odd = include.odd %}{% endif %}

It would be good for the default filter to support include params. I got then do something like:

{% assign odd = include.odd | default:  page.odd | default:  site.odd | false %}

Although this example is a little contrived, I'm happy to provide a more real-world example.

Start by locating the default filter implementation and the handling of missing include parameters in the Liquid codebase. Review existing filter tests and add coverage for defaulting a missing include parameter to a fallback value. Done means the shown include-based default workflow no longer errors and the behavior is covered by tests.