[tools] Print stdout header even when the output is empty

This will allow us to determine whether tests are not printing anything or
whether our infrastructure is failing to retrieve the stdout.

TBR=machenbach@google.com

No-Try: true
Bug: chromium:891314
Change-Id: I0786e77b4db9f247e02a25924acd6d6773c0db9f
Reviewed-on: https://chromium-review.googlesource.com/c/1282962
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56701}

tools/run_perf.py

@@ -783,7 +783,7 @@ class AndroidPlatform(Platform):  # pragma: no cover
       logging.info(title % "Stdout" + "\n%s", e.output)
       raise
     except android.TimeoutException as e:
-      if e.output:
+      if e.output is not None:
         logging.info(title % "Stdout" + "\n%s", e.output)
       logging.warning(">>> Test timed out after %ss.", runnable.timeout)
       stdout = ""