xapi: handle requests without durations

Change-Id: I907e2131ad01e4aa9dda377c8aadf3f53eac5017
Reviewed-on: https://gerrit.instructure.com/42864
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Braden Anderson <braden@instructure.com>