[buildbucket] Handle absence of step end time
[buildbucket] Handle absence of step end time

If a step does not have end time, it cannot have a terminal status.

R=5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org, 9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug: 843714
Change-Id: Id306bd9f6d066ba29b390548feebf449be38aa29
Reviewed-on: https://chromium-review.googlesource.com/c/1334208
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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