Initialize known collection types to empty instead of null

[cushon]

No description provided.

[JakeWharton]

What about @Nullable properties. Should we allow them to be initialized to null since you've essentially opted into the behavior?

[cushon]

Should we allow them to be initialized to null since you've essentially opted into the behavior?

SGTM, I updated the pull request.

[rharter]

Thanks @cushon, this looks great. I was initially thrown by the Immutable* additions, but that will only become a dependency if the app is already using Guava, so nothing is imposed on the user.

[rharter]

@hzsweers has raised a valid concern that this breaks the AutoValue expectation that null collection and empty collections are different. If your collection property is non-nullable, then it should exist in your JSON. (i.e. if you deserialize JSON with this change, then serialize JSON it would be different).

I've opened issue #73 to discuss this.