fix alerts.js error with non-existent roles
fix alerts.js error with non-existent roles

test plan:
* create a custom account role
* enable alerts for the account
* create an alert referencing the custom role
* delete the custom role
* visit the accounts settings page using optimized js
e.g. "accounts/:id/settings?optimized_js=1"
* settings should still load

fixes #CNVS-12655

Change-Id: I0b9d86e8d331189836e5def4407fb155003cf8d7
Reviewed-on: https://gerrit.instructure.com/33961
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Clare Strong <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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