The step_result json can currently be empty if the orchestrator build
is canceled while running bot_update. Trying to do
step_result.json.output in the finally block raises an "Uncaught
exception AttributeError".
Ex: https://ci.chromium.org/ui/p/chromium/builders/try/win10_chromium_x64_rel_ng/1121568/overview

This change checks for the json attr before trying to read json.output.

Bug: 1305332
Change-Id: I1dc277642ece1b96144be6381a7c4b8426cb2bc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3518372Reviewed-by: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>