mapOrAsyncForNullable

Return the result of transformer with using input as an argument for it if input is not null. Otherwise, return defaultValue.

Basically, this operation is a combination map() and unwrapOr().

• U must not be Nullable<*>.
  • If the result of transformer is null, this throw an Error.
  • If the result of defaultValue is null, this throw an Error.
• If you'd like to accept Nullable<*> as U, use a combination andThen() and or().