Optional relayState during logout

[m0ark]

According the the specification relayState is optional.
I.e. it is not used in iframe logout scenarios coming from SimpleSAMLphp itself.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.80%. Comparing base (c1b753d) to head (ee8e9fc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master     #37      +/-   ##
===========================================
- Coverage      0.80%   0.80%   -0.01%     
- Complexity      155     156       +1     
===========================================
  Files             7       7              
  Lines          1976    1979       +3     
===========================================
  Hits             16      16              
- Misses         1960    1963       +3     

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