Do not fall through to error case after successful SAML login

[knolleary]

Fixes #5856

Adds a missing return to avoid falling through to logging an invalid error.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.48%. Comparing base (55c639b) to head (17f4792).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
forge/ee/routes/sso/auth.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5857      +/-   ##
==========================================
- Coverage   76.49%   76.48%   -0.01%     
==========================================
  Files         369      369              
  Lines       17969    17970       +1     
  Branches     4189     4189              
==========================================
  Hits        13745    13745              
- Misses       4224     4225       +1     

Flag	Coverage Δ
backend	76.48% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.