[milo] initialize step to empty slice
[milo] initialize step to empty slice

Avoid null slices in buildbot structs because they are converted to JSON null
instead of JSON array.

Bug: 801052
Change-Id: I0908415cc1ee8b90d70d77d8eba8f9b908a65965
Reviewed-on: https://chromium-review.googlesource.com/862608
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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