deal with nil position in module item sequence API

test plan:
 - set a module item's position to nil via the console
   (I don't know how this happened in the real world; the ticket
    mentions a course copy but I was not able to reproduce the
    problem by copying the referenced course.)
 - the prev/next buttons should still show up when viewing module
   items (no 500 error when making the module_item_sequence call)

fixes CNVS-14671

Change-Id: I95340006c44a04cbffa608af62a98fd2653a363c
Reviewed-on: https://gerrit.instructure.com/39057
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>