Dev fix: The API chaining was a little broken if the inner function
returned undefined, null etc.

- The base issue was in the instanceof check which has a priority error,
  it was:
    ! obj instanceof _Api
  which is the same as:
   (! obj) instanceof _Api
  which is rubbish.

  We want:
    ! (obj instanceof _Api)
  but there is of course a wrapper function, so that needs ot be taken
  into account as well. The new logic does just that.