value_to_array returns empty array for nil

test plan
- run specs

Change-Id: I185717113fe0f6e49c7cae800597ea530d4f6a61
Reviewed-on: https://gerrit.instructure.com/30812
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>