Avoid a JS error on pages with no nav
Avoid a JS error on pages with no nav

This will avoid a JS error in the console on the 404 page as well as the
“Masquerade as” page (or any other page that doesn’t render the Global
Nav but does js_bundle the “navigation_header” bundle. The error wasn’t
actually breaking anything but it was getting logged.

Test plan:
* as an admin
* go to /courses/x/?become_student
* it should render the “become user” page and there should be no JS
  JS error logged to the console

Change-Id: I57006e233a65922654c5e7833de8bc3ae935fca3
Reviewed-on: https://gerrit.instructure.com/205613
Tested-by: Jenkins
Reviewed-by: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

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