Fix handling for nil pointers given user-provided output schema

[findleyr]

Following up on some discussion in #685: When fixing #199, I was overly cautious about handling typed nils: if the user provided their own output schema, we wouldn't fix typed nils for them, and so returning 'nil' would result in a missing structured output, rather than its zero value.

This was wrong: switching from an inferred schema to a user-provided schema shouldn't change the handling of nil, and structured output, if provided, must be an object.

PR incoming.