Fix date showing for students without last activity
Fix date showing for students without last activity

fixes VICE-2212
flag=none

Test Plan:
 - Create a new course
 - Add a new enrollment to the course
 - View Users page
 > Should see an empty last activity field for the new user

Change-Id: I22695e28a49c0c518837b68b5f13d3290e75a610
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/277050
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Product-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms