Void argument routes should not require `null`

When calling an endpoint that does not accept any parameters (e.g. usersGetCurrentAccount()), null must be passed as an argument. This is somewhat confusing and the error message returned when you forget to pass null doesn’t help either:

> dbx.usersGetCurrentAccount()
POST https://api.dropboxapi.com/2/users/get_current_account 400 (Bad Request)
Error in call to API function "users/get_current_account": request body: could not decode input as JSON
> dbx.usersGetCurrentAccount(null)
lib$es6$promise$promise$$Promise {xot8798dunmi: 2, _state: undefined, _result: undefined, _subscribers: Array[0]}

We should default to sending null when no argument is supplied.

[bobmoff]

The issue still remains, in the typescript definitions.

[greg-db]

This should be fixed for TypeScript as of the latest version, v7.0.0.