[buildbucket] Handle nil history options

Bug: 1042991
Change-Id: I512a66660e58ee3bbff735f93b1eac1cb58f254f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2864511
Reviewed-by: Scott Lee <ddoman@chromium.org>
Reviewed-by: Yuanjun Huang <yuanjunh@google.com>
Commit-Queue: smut <smut@google.com>
diff --git a/buildbucket/appengine/internal/resultdb/resultdb.go b/buildbucket/appengine/internal/resultdb/resultdb.go
index cca5416..1245d55 100644
--- a/buildbucket/appengine/internal/resultdb/resultdb.go
+++ b/buildbucket/appengine/internal/resultdb/resultdb.go
@@ -79,7 +79,7 @@
 						ProducerResource: fmt.Sprintf("//%s/builds/%d", bbHost, b.Proto.Id),
 						Realm:            realm,
 						HistoryOptions: &rdbPb.HistoryOptions{
-							UseInvocationTimestamp: cfg.Resultdb.HistoryOptions.UseInvocationTimestamp,
+							UseInvocationTimestamp: cfg.Resultdb.HistoryOptions.GetUseInvocationTimestamp(),
 						},
 					},
 					RequestId: invID,