fix(backends): avoid crash on invalid e-mail address

[nijel]

This PR applies 3/3 suggestions from code quality AI findings.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.39%. Comparing base (06e154a) to head (87e79d0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/base.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1809      +/-   ##
==========================================
- Coverage   84.41%   84.39%   -0.02%     
==========================================
  Files         342      342              
  Lines       12593    12596       +3     
  Branches      607      608       +1     
==========================================
+ Hits        10630    10631       +1     
- Misses       1742     1743       +1     
- Partials      221      222       +1     

Flag	Coverage Δ
unittests	84.39% <60.00%> (-0.02%)	⬇️

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

☔ View full report in Codecov by Harness.
📢 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.