[Telemetry] Allow missing results in CSV output.

It is only a problem if a page tries to output more results than we are
expecting.

BUG=

Review URL: https://codereview.chromium.org/137603005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245721 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245721