Currently, when a request body is empty, the JSON request validator
would parse it into None, which is later passed down to the JSON Schema
validator. However, jsonschema's validation error message for this case
(when nullable is false) "None is not of type 'object'" is not
particularly friendly to either the API user, nor the website developer.

This change adds a specific check before the None value is passed to
jsonschema to emit a better error message directly.

I also added some drive-by improvements on function argument typing
since _validate in validators don't seem to expect receiving None, but
_parse (the result of which is passed to _validate) is totally allowed
to return None (or anything really). This does not seem to reflect the
logic well.