@Nullable
    @Override
    public Boolean isResponseStreaming(Type returnType, @Nullable MediaType contentType) {
        return null;
    }
A response converter implementation which creates an {@link HttpResponse} of {@link HttpStatus#NO_CONTENT}
when the specified {@code result} is {@code null}.
core/src/main/java/com/linecorp/armeria/server/annotation/NullToNoContentResponseConverterFunction.java