Fix crash in the flakiness dashboard server.

In theory, this case should never happen, but clearly there's
a bug somewhere. I don't have time to dig into the bug right now,
so just bandaid over it by returning the empty string. This will
mean the old data is lost, since we basically treat this as an
empty file, but at least we start collecting the new data.

We'll still log the error though, so we can grep the log to find
these cases in the future and fix them.

BUG=377594
NOTRY=true

Review URL: https://codereview.chromium.org/298163003

git-svn-id: svn://svn.chromium.org/blink/trunk@174990 bbb929c8-8fbe-4397-9dbb-9b2b20218538