Add default values and fix empty descriptions renderings

[treble-snake]

This PR contains 2 changes:

First, if an item or a node doesn't have a description, the actual value in the AST might be null. There was a small bug about not casting the value to an empty string, so null would be rendered on the web page.

Second, arguments and input values might have a default value, which I believe should be reflected in the docs. I added a simple piece of code rendering that like limit: Int = 10.

Cheers!

Related issues: #53