fix caching issue when moving module item into an empty module

if you drag a module item into an empty module, the module is
currently not being touched after the module item is saved,
leaving the cached html content for the module showing no
module items

fixes #CNVS-12460

Change-Id: I4820018fd52e3862a9b891fa2bbe2d15b4905df1
Reviewed-on: https://gerrit.instructure.com/33554
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>