I think that because of the change to use StructureObject the 'tests' key is contains None rather than being absent. (in the input to ProcessSwarmingTasksResultPipeline see https://findit-for-me.appspot.com/_ah/pipeline/status?root=b5cf1fb2dc3c4d05845fed0263c2dfae#pipeline-b5cf1fb2dc3c4d05845fed0263c2dfae for example)

I am making a change to treat the key containing None the same as if the key was absent.

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/8b0834e806cd654243ba4021f33c6724e127392f

commit 8b0834e806cd654243ba4021f33c6724e127392f
Author: Roberto Carrillo <robertocn@google.com>
Date: Wed Dec 06 04:50:54 2017

[Findit] Treat an empty 'tests' key same as absent.

R=wylieb,stgao,lijeffrey
BUG= 792292 

Change-Id: I98d183a800d1967b1233e318dc9dfa91f3cbd372
Reviewed-on: https://chromium-review.googlesource.com/810035
Reviewed-by: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Commit-Queue: Shuotao Gao <stgao@chromium.org>

[modify] https://crrev.com/8b0834e806cd654243ba4021f33c6724e127392f/appengine/findit/waterfall/process_swarming_tasks_result_pipeline.py