[cv] tiny fix to avoid spurious diffs between nil and empty slice.
[cv] tiny fix to avoid spurious diffs between nil and empty slice.

R=yiwzhang,ddoman

Bug: 1227384
Change-Id: Ieddf46602dd42d65ba6c4fa2424a26e68efa70e1
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3015840
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go