[java][native] fix empty response body

[Frontrider]

To fix #13968.

The issue was that the client did not check if the response was empty before trying to read it as json, but this was only a problem if the endpoint could respond with an object and an empty response as well with the most common case being a 204 response.
This bug does not affect the async client, only the sync client.

The documentation for what I did is in a comment above the change in api.mustache.
https://github.com/telekom/openapi-generator/blob/fix/13968/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache#L279
I included it there, because I see that there was already a failed attempt to do it in #13993, and the reason why that code could not work is not obvious.

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[wing328]

thanks for the PR.

can you please PM me via Slack when you've time?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g