[BUG] Java generated client throws MismatchedInputException when response body is empty

Description

When server returns 2xx status code, generated client code tries to parse body even when Content-Length is specified 0 in header. This in turn throws "com.fasterxml.jackson.databind.exc.MismatchedInputException: No content to map due to end-of-input".

Example of generated code:

public ApiResponse<Object> executeCallUsingPOST2WithHttpInfo(InvokeData data) throws ApiException {
    HttpRequest.Builder localVarRequestBuilder = executeCallUsingPOST2RequestBuilder(data);
    try {
      HttpResponse<InputStream> localVarResponse = memberVarHttpClient.send(
          localVarRequestBuilder.build(),
          HttpResponse.BodyHandlers.ofInputStream());
      if (memberVarResponseInterceptor != null) {
        memberVarResponseInterceptor.accept(localVarResponse);
      }
      try {
        if (localVarResponse.statusCode()/ 100 != 2) {
          throw getApiException("executeCall", localVarResponse);
        }
        return new ApiResponse<Object>(
          localVarResponse.statusCode(),
          localVarResponse.headers().map(),
          memberVarObjectMapper.readValue(localVarResponse.body(), new TypeReference<Object>() {}) // closes the InputStream
          
        );
      } finally {
      }
    } catch (IOException e) {
      throw new ApiException(e);
    }
    catch (InterruptedException e) {
      Thread.currentThread().interrupt();
      throw new ApiException(e);
    }
  }

openapi-generator version

6.2.0

Steps to reproduce

Make a successful API call which returns no content

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/7720

Suggest a fix

Don't parse response with Jackson if content length is 0.

Start by reproducing a successful API call with a 2xx response and an empty body, then trace the generated Java client code around the Jackson readValue call shown in the issue. Done means empty successful responses no longer raise MismatchedInputException while non-empty responses still parse normally; add or update a regression test if the relevant test location is found.