[buildbucket] Set empty result details to nil
[buildbucket] Set empty result details to nil

The type-specific 0 value (an empty string) is invalid in Python, which
expects either None or a valid JSON-encoded dict.

Bug: 1042991
Change-Id: Ic80beef84377f0857d6d439caf307ea7334b60d2
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2322177
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

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