Fix RP Initiated Logout when no user is found

[tonial]

Fixes #1280

Description of the Change

Don't crash when we only have a AnonymousUser to logout (no id_token_hint or existing session),

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #1284 (e5e0ff0) into master (2f3dd45) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head e5e0ff0 differs from pull request most recent head 1c44a29. Consider uploading reports for the commit 1c44a29 to get more accurate results

@@            Coverage Diff             @@
##           master    #1284      +/-   ##
==========================================
- Coverage   97.30%   97.25%   -0.05%     
==========================================
  Files          32       32              
  Lines        2003     2005       +2     
==========================================
+ Hits         1949     1950       +1     
- Misses         54       55       +1     

Impacted Files	Coverage Δ
oauth2_provider/views/oidc.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[n2ygk]

LGTM. Thanks!