[MODEL] Fix import when preprocess returns empty collection

[timbleck]

Closes #719 and also fixes existing test on preprocess which had no effect.

[aeroastro]

Great work!
I have been using 5.x version on production environment with a similar patch.
I'm wondering when this will be merged into all the elasticsearch-rails versions.

[aeroastro]

@karmi

Could you take a short look into this?

[karmi]

@timbleck , @aeroastro , sorry for the long silence here! As I've mentioned on other tickets, we're scheduling a significant effort to give attention to the elassticsearch-rails groups of libraries again in couple of weeks, so I'd rather not merge the PR now, but wait a bit, until we have more capacity and focus to be more organized with that...

[estolfo]

thanks @timbleck !