[swarming] Don't use null user/tags in test data
[swarming] Don't use null user/tags in test data

As of I150235a44b0adc347817f9e5cc22c5469ceaab03, the with_user and
with_tags methods of swarming.TaskRequest no longer accept None. So
change the test data so that they don't get called with None. (This
should've been done in the same CL.)

Recipe-Nontrivial-Roll: fuchsia
Bug: fuchsia:40002
Change-Id: I199c10661b9488271dcadafa56e04717dc8909c0
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1931224
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Oliver Newman <c98d05fdc4005e0f75e160701d6b971b76237a97@google.com>

luci/recipes-py,luci/recipes-py