Currently Errai is using GWT 2.8.0-beta1 which does not have java.util.Optional, so there is no special handling of Optional. There won't be any special integration in of Optional in Errai until there is a new GWT release containing it.
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Please see JDK emulation section of the change log for GWT 2.8.0 RC1 (pulished today).
The Java8 Optional feature has been introduced (Emulate Optional<T> and its int, long, double variants).
So it looks like it solves the issue
Gwt 2.8.0-RC1 already provides emulation for Optional