handle swagger exception for array objects

When docs have an 'array' type that has no 'items', it is causing
an exception that prevents the Swagger UI from loading. This patch
will fix it and help diagnose future issues.

Fixes SIS-351

Change-Id: Ia41299cbcfb0e6e1efe77b21ea7427ab65e51999
Reviewed-on: https://gerrit.instructure.com/37529
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>