avoid warnings from Bluebird when in "debug" mode
avoid warnings from Bluebird when in "debug" mode

- if resolving a promise with bluebird's `settled()`, "debug" mode is enabled
- if "debug" mode is enabled, and `then()` is not passed a function (instead, `undefined`), bluebird emits a warning:

  ```
  Warning: .then() only accepts functions but was passed: [object Undefined]
  ```
- instead of passing `undefined` to `then()`, use an identity function.

vstirbu/fsm-as-promised,boneskull/fsm-as-promised