One-liner fix of issue 23514, allowing Kotlin 2.x to build generated code.

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
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fixes #23514
This is my first contribution, so any feedback is much appreciated.
@dennisameling, @e5l, @stefankoppier, @yutaka0m, @4brunu, @andrewemery, @Zomzog, @karismann

Summary by cubic

Handle nullable OkHttp response.body in the Kotlin jvm-okhttp ApiClient.responseBody to prevent NPEs and Kotlin 2.x compile errors. Fixes #23514.

• Bug Fixes
  • In kotlin-client/libraries/jvm-okhttp ApiClient.kt.mustache, return null when response.body is null in responseBody.
  • Regenerated Kotlin jvm-okhttp sample clients to reflect the template change.

^{Written for commit 5d16e35. Summary will update on new commits.}