Handle nullable enum types

[bespokebob]

Nullable primitive types were being mapped correctly, but other value types like Enums were not. Make nullable types map to their underlying type, and generate the schema for nullable Enums to be the same as the underlying Enum as well.

[yahehe]

Looks fine to me, @jnallard you touched this most recently

[jnallard]

Looks fine to me too! I'll merge it in