use default syllabus text for immersive reader
use default syllabus text for immersive reader

closes MAT-479
flag=more_immersive_reader

Test Plan
- Merge the commit.
- Verify that Immersive Reader works for the syllabus when it only
  has the default syllabus text.
- Verify that Immersive Reader works for the syllabus when a user
  has added text to the syllabus.

Change-Id: If52e67240b154277b6029d93e60ae6ae52daee86
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/275949
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Product-Review: Gary Mei <4dff5ebcff9ed76793b70b9470ac64791a16ed11@instructure.com>
Reviewed-by: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
QA-Review: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>

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