Store nil for default targets - only fill in the target in GetAll
Store nil for default targets - only fill in the target in GetAll

BUG=

Review URL: https://codereview.chromium.org/1731123002

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