style(StudentContextTray) allow initials if no image for avatar
style(StudentContextTray) allow initials if no image for avatar

Fixes: UIDEV-216

test plan:
- within a course select 'people' from the sub-nav
- click on one of the users names that has not uploaded an image
  for their avatar ... initials should show in the student tray
  example: https://cl.ly/3d531d6cad0c
- click on a user that has uploaded an image to ensure it still
  shows in the student tray

Change-Id: Ie950231f50c428ad029b13042e3a3261759ec951
Reviewed-on: https://gerrit.instructure.com/164710
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Mary Jane Anderson <bfb9d3817d27ddfdcd2a6c0c878e2771b3b2c4a7@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms