StreamName was annotated as @NonNull

@NonNull
public String getName() {
    return name;
}

This is actually correct, but under the covers...

• The builder allowed null but this was handled in JetStreamManagement when adding or updating a stream, the name was valdiate. This has been moved to the builder. Same type of exception and same message, so is backward compatible, and should have no practical issues for users, since it's still the same error, just on build(...) instead of addStream or updateStream, which is called with a built StreamConfiguration.
• The StreamInfo creation always read the "config" field from an incoming response to getStreamInfo(). This was a problem because if the request was an error, there is no StreamConfiguration in the response. So this is fixed by checking the field first.