Support an option to not allow explicit null values for nullable fields.

[jakemac53]

See InputSet as an example. Both of its fields are allowed to be null, but we don't really want users to be able to specify them as null explicitly in yaml. For instance, this should give an error:

targets:
  $default:
    sources:
      include:

include: there isn't treated any differently than if it didn't exist at all, and should probably be an error.

[kevmoo]

This is a weird angle on required, but orthogonal. Something like disallowNull or mustContainValue?

It'd be pretty easy to roll into $checkKeys