bugfix for course progress api (fixes CNVS-12106)

Issue:
  * When creating a module, then a new assignment under that module,
    the ContextModuleProgression.requirements_met field comes back
    as nil, causing course_progress_json to bomb. The previous code
    assumed an array would always be returned, but this is not the case.

Solution:
  * Cast requirements_met to an array.
    + nil.to_a -> []
    + [1].to_a -> [1]

Test plan:
  * Test against Gallery course progress commit (g/32571) with some new modules
    and blank assignments created through the module interface.

Change-Id: Ie666fcc0031009b5ff32ad151659cb2818bf7b6e
Reviewed-on: https://gerrit.instructure.com/32570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>
Reviewed-by: Michael Ziwisky <mziwisky@instructure.com>