fix importing module items with missing content

fixes an issue that arose with rails 3, where
module items that were not set with content (and thus
shouldn't have been saved) were being autosaved and
breaking the entire modules page

test plan:
* import the package(s) referenced in the ticket
* the modules page should still be functional

fixes #CNVS-14777

Change-Id: I82fe287a4db9492489461e971911a0f7c15fbcb0
Reviewed-on: https://gerrit.instructure.com/39404
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>