Make sure callback happens if a model is not attached to the data source

/to @bajtos or @ritch 

The PR fixes a hang issue during `automigrate` if one of the models is not attached to the data source.

I'll add a test in connectors.
`async` invokes the callback function synchronously if the array is empty. See https://github.com/caolan/async/blob/master/lib/async.js#L145.