fix: Fail safe, if optional endpoints are not given

[jkroepke]

If discovery document doesn't contains some optional endpoints, calling some endpoints functions may fail hard

Example with end session

Before:

Post "": unsupported protocol scheme ""

After:

end session endpoint not set

Definition of Ready

• I am happy with the code
• Short description of the feature/issue is added in the pr description
• PR is linked to the corresponding user story
• Acceptance criteria are met
• All open todos and follow ups are defined in a new ticket and justified
• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
• My code has no repetitions
• Critical parts are tested automatically
• Where possible E2E tests are implemented
• Documentation/examples are up-to-date
• All non-functional requirements are met
• Functionality of the acceptance criteria is checked manually on the dev system.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 62.03%. Comparing base (0992c5f) to head (1c66947).
Report is 40 commits behind head on main.

Files	Patch %	Lines
pkg/client/client.go	16.66%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #582      +/-   ##
==========================================
+ Coverage   60.06%   62.03%   +1.97%     
==========================================
  Files          80       80              
  Lines        6998     6169     -829     
==========================================
- Hits         4203     3827     -376     
+ Misses       2498     2037     -461     
- Partials      297      305       +8     

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[github-actions]

🎉 This PR is included in version 3.20.1 🎉

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