fix syntax error in module_item_json

and add a test that covers this code path

test plan:
 - add an external tool module item (such as YouTube)
 - verify that the module item sequence ("Previous" and "Next"
   buttons) continues to work

fixes CNVS-14898

Change-Id: Iaf53a5d17f3ad8e1c71eea1e30d04bec5dc465ae
Reviewed-on: https://gerrit.instructure.com/39577
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>