Original PR and conversation: https://github.com/resteasy/resteasy/pull/3030
This is a breaking change, it's probably a starting point. Probably a configuration is needed to opt-in for the feature.
Returning null here doesn't seem correct at all. Determining a way to reproduce this would be ideal to see what happens. Not having a media type in general seems like a bug.
If the server doesn't send the Content-Type header, the client gets null with no explanation. AFAIU, the only way to troubleshoot is debugging.