optimize due date cacher trivial case

Change-Id: I238bd25f5083832abb5656580855ba8a64b1a54b
Reviewed-on: https://gerrit.instructure.com/27092
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>