Allow empty records for JSON generation

[jaredponn]

This PR removes the restriction of generating code instances for only non empty JSON objects.

Indeed, the empty JSON object

{}

is valid JSON, so schemas like

record MyEmptyRecord = { }

derive Json MyEmptyRecord

should be valid.

[bladyjoker]

record MyEmptyRecord = { }

Is not something LB can swallow at the moment. Why? I don't recall exactly, but it was a decision that spanned parts from the Compiler to the Codegen. Nowadays, I think we error on empty records, and we have no syntax for empty products.

However, I don't think this change is problematic. If anything, when we introduce empty product/records this would then just work.