fix: mark identity as optional in session struct

[aeneasr]

The identity is not always available in the session struct, for example when AAL2 is required.

Closes #3461

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
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• I have added tests that prove my fix is effective or that my feature
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Further Comments

[codecov]

Codecov Report

Merging #3463 (db92322) into master (dfcbe22) will increase coverage by 0.10%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head db92322 differs from pull request most recent head acca744. Consider uploading reports for the commit acca744 to get more accurate results

@@            Coverage Diff             @@
##           master    #3463      +/-   ##
==========================================
+ Coverage   78.11%   78.21%   +0.10%     
==========================================
  Files         339      339              
  Lines       22463    22463              
==========================================
+ Hits        17547    17570      +23     
+ Misses       3602     3583      -19     
+ Partials     1314     1310       -4     

Files Changed	Coverage Δ
session/session.go	78.90% <ø> (ø)

... and 2 files with indirect coverage changes