[BUG] Assertion error when handling a 500 from server

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

I am using the openapi-generator-cli to generate java code. The generated code throws an assertion error while attempting to deserialize an empty response body coming from a 500 response.

openapi-generator version

7.8.0

OpenAPI declaration file content or url

Generated code will call getApiException when server returns a 500:

    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("something", localVarResponse);
        }

The generated code for getApiException is the following:

  protected ApiException getApiException(String operationId, HttpResponse<InputStream> response) throws IOException {
    String body = response.body() == null ? null : new String(response.body().readAllBytes());
    String message = formatExceptionMessage(operationId, response.statusCode(), body);
    return new ApiException(response.statusCode(), message, response.headers(), body);
  }

The response.body() is not null so the code will attempt to read the bytes of an empty response and trigger an assertion in the underlying int read(byte[] bytes, int off, int len) method:

  public int read(byte[] bytes, int off, int len) throws IOException {
   ...
   assert read > 0 && read <= buffer.remaining();    
   ...
   }    

Generation Details

openapi-generator-cli generate --generator-key=client-java

Steps to reproduce

Create an endpoint that trhows a 500 with a empty body response.

Related issues/PRs

N/A

Suggest a fix

Do not attempt to deserialize the bytes if the response is null not null but empty.

Start with the client-java generation entry point from openapi-generator-cli generate --generator-key=client-java and inspect the generated getApiException path shown in the report. Reproduce a 500 response with an empty body, then add coverage for that case and verify that generation and the resulting request handling no longer trigger the assertion.