telemetry: Fix EmptyTraceImporter

The EmptyTraceImporter was actually broken by https://codereview.chromium.org/183683003/.
This patches fixes it and improves test coverage.

R=nduca@chromium.org, dtu@chromium.org, tonyg@chromium.org
BUG=347802

Review URL: https://codereview.chromium.org/184873007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254387 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254387