`next.body()` raises an error when not called in an inherited context

Migrated issue, originally created by jvanasco (@jvanasco)

I noticed this while bugfixing some parent templates in an inheritance chain.

The following is raised when the template containing ${next.body()} is called directly:

AttributeError: 'builtin_function_or_method' object has no attribute 'body'

While this makes sense, I think there are better ways to handle it:

a_ raise a custom error that says "You're calling this direct, must inherit"
b_ [preferred] just return an empty string value when there is no next

I think the latter behavior is better for introductory users.

Start by reproducing the direct template call containing ${next.body()} described in the issue, then trace how next.body() is resolved outside an inheritance chain. The change is done when that direct call returns an empty string, with a regression test covering the behavior.