[bb] Fix String method for messageSliceFlag when flag is zero value
[bb] Fix String method for messageSliceFlag when flag is zero value

Before this change, if the flag is zero value, the String method will
error out because it is calling Len() on a zero value of reflect.Value.

R=nodir

Bug: 1039823
Change-Id: I913f89696978ab1da011e2518838abcddbf955e0
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2048215
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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