No bug: [diffoscope] Output an empty diff if the files do not differ; r=glandium

Currently, if the files match and you try to look at the diff, you get

   {
       "reason": "file-missing-on-worker",
       "message": "Artifact \"public/diff.html\" not found at \"/builds/worker/diff.html\""
   }

which makes it hard to tell if there was an error generating a diff, or if the files matched.
This changes things to ask diffoscope to always output a diff, to remedy that confusion.

Differential Revision: https://phabricator.services.mozilla.com/D24316

--HG--
extra : moz-landing-system : lando

taskcluster/ci/diffoscope/kind.yml

@@ -23,6 +23,7 @@ transforms:
 # generated for each build.
 job-defaults:
   args: >-
+    --output-empty
     --no-default-limits
     --max-page-size 100000000
     --max-page-diff-block-lines 10000