[buildbucket] Don't nil entity inside transaction
[buildbucket] Don't nil entity inside transaction

Bug: 1189958
Change-Id: I7b4e1dcbc5a435506428b1e7c95ffdf33a91ff89
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2773836
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>

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