fix: prevent crashes by filtering out null auth providers

[ewan-escience]

Prevent crashes when auth provider down

Changes proposed in this pull request

• Do an extra null check to prevent crashes when an auth provider is down

How to test

• Have LinkedIn auth properly setup in your .env
• Simulate LinkedIn being down by adding 120.0.0.1 www.linkedin.com to /etc/hosts
• docker compose down --volumes && docker compose build --parallel && docker compose up --scale data-generation=0
• Visit http://localhost/, because of the line in /etc/hosts, wait 10 seconds
• Click on Sign in, the LinkedIn option should not be there
• Remove the line from /etc/hosts
• Restart the RSD and refresh the page
• LinkedIn should be back as a signing in option
• (Optional): try this out with other auth providers as well

Note: In order to get this fix out, I skipped the "greying out" part I suggested in #1367.

Closes #1367

PR Checklist:

• Increase version numbers in docker-compose.yml
• Link to a GitHub issue
• Update documentation
• Tests

[sonarqubecloud]

Quality Gate passed for 'rsd-frontend'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud