[JENKINS-76231] Fix NullPointerException when Content-Type header is missing

[Fluder-Paradyne]

Added a null check in ResponseContentSupplier.readCharset() to handle responses without a Content-Type header. Previously, the code would throw a NullPointerException when processing 204 No Content responses (such as those from GitHub Actions API) because it attempted to call .getValue() on a null header. The fix now checks if the Content-Type header exists before accessing its value, allowing these responses to be processed successfully.

Also added a test case to verify that responses without Content-Type headers are handled correctly.

Testing done

Ran the tests without the fix and the test failed.

Issue link

https://issues.jenkins.io/browse/JENKINS-76231

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[MarkEWaite]

Thanks very much! I confirmed that the test shows the bug when the main code is not included and the test passes when the main code is included.