SAM-2301: Assignment delivery throws error when time limit set to 'null'

git-svn-id: https://source.sakaiproject.org/svn/sam/branches/sakai-10.x@307041 66ffb92e-73f9-0310-93c1-f5514f145a0a
SAM-2297: Time remaining does not show on Table of Contents

git-svn-id: https://source.sakaiproject.org/svn/sam/branches/sakai-10.x@307042 66ffb92e-73f9-0310-93c1-f5514f145a0a