swarming: return result always when creating a new task
swarming: return result always when creating a new task

This is to return invocation ID.

Bug: 1065139
Change-Id: I3b357c413c89535463f5d3d2a9b457a8e9142b4e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2206766
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py