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

• [x] I am happy with the code
• [x] Short description of the feature/issue is added in the pr description
• [ ] PR is linked to the corresponding user story
• [ ] Acceptance criteria are met
• [x] 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.
• [x] No debug or dead code
• [x] 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
• [x] Functionality of the acceptance criteria is checked manually on the dev system.

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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[bot]]

:tada: This PR is included in version 3.20.1 :tada:

The release is available on GitHub release

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