[JAVA] NullPointerException from generated Java OAuth code when the OAuth server returns no body

Description

When making a HTTP request using the Java code that Swagger generates, and the OAuth server returns a response with no body - e.g. a HTTP 401 Unauthorized response without a body, then the generated code throws a NullPointerException.

The NullPointerException hides the nature of the error (in my case, that the username and password I am trying to use are invalid) and therefore makes debugging harder.

Swagger-codegen version

3.0.13

Suggest a fix/enhancement

The problematic code is here: https://github.com/swagger-api/swagger-codegen/blob/4607a90d7b69463a0ae8fc94fac68fc95d80965e/modules/swagger-codegen/src/main/resources/Java/libraries/feign/auth/OAuth.mustache#L174

feignResponse.body() can be null if the HTTP response did not include a body.

The ideal fix for me would involve detecting HTTP 401 (or possibly, all non-200) responses and throwing an exception with a error message that describes what response code was returned.

Start at modules/swagger-codegen/src/main/resources/Java/libraries/feign/auth/OAuth.mustache around line 174, where feignResponse.body() can be null. Trace how empty-body OAuth responses are handled and verify the generated Java client no longer throws a NullPointerException. Done should preserve a useful error describing the returned HTTP response, especially a 401 response.