Null default for empty collections is annoying

It's very annoying to have to write:

```java
@AutoValue.Builder
public abstract static class Builder {
public static Builder create() {
return new AutoValue_MyType.Builder()
.setCharges(emptyList())
.setPayments(emptyList())
.setParticipants(emptyList())
.setTags(emptyList());
}
```

just to ensure that a field `@Nonnull public abstract List getCharges()` defaults to empty-list on the builder instead of null. It would be nice to have some way to specify a default for non-primitive non-null types.