Validating a struct is ambiguous, because a zero field could be
considered missing or present.
Interpret a zero optional struct field in whichever way results in
success.
Change-Id: I6a9474e6f6558d2a8522bc5ba0451967367e467d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/675956
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>