Add empty JSON as a valid empty type

[ramongr]

Adds a new empty type called Blueprinter::EMPTY_JSONB to represent possible empty values from jsonb columns in databases.

[philipqnguyen]

Correct me if I'm wrong, the most likely scenario for using JSONB is when there is a JSONB column in a postgresql column. However, when you access that corresponding attribute in Rails, it would already be deserialized into a Hash right? So I don't see why we need a JSONB type here.