Issue Summary
After upgrading from redash:latest SAML login fails.
In the browser I see -
Internal Server Error
The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.
Looking in the logs revealed an error -
[2021-02-06 12:56:30,602][PID:10][ERROR][redash.app] Exception on /saml/callback [POST]
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1951, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask_restful/__init__.py", line 269, in error_router
    return original_handler(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1820, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1949, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1935, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/app/redash/authentication/saml_auth.py", line 85, in idp_initiated
    authn_response.ava["FirstName"][0],
KeyError: 'FirstName'
Technical details:
- Redash Version: redash/redash:9.0.0-beta.b42121
 - Browser/OS: Chrome/MacOS 10.14.6
 - How did you install Redash: K8S
 
