[milo] make CurrentBuilds empty array
[milo] make CurrentBuilds empty array

nil in JSON is not an array, so avoid using nil slices in structs that get
converted to JSON.

TBR=iannucci@chromium.org
Bug: 793514
Change-Id: Id11b0a2dda45534ba12bc42bcb5bdc5056e793f9
Reviewed-on: https://chromium-review.googlesource.com/833453
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@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