`rust-server-codegen`: don't check JSON Content-Type header if there is no body (#888)

        } else {

            """

            async fn from_request(req: &mut #{Axum}::extract::RequestParts<B>) -> Result<Self, Self::Rejection> {

                #{SmithyHttpServer}::protocols::check_json_content_type(req)?;

                Ok($inputName(#{parse_request}(req).await?))

            }

            """.trimIndent()

                """

                let body = request.take_body().ok_or(#{SmithyHttpServer}::rejection::BodyAlreadyExtracted)?;

                let bytes = #{Hyper}::body::to_bytes(body).await?;

                if !bytes.is_empty() {

                    #{SmithyHttpServer}::protocols::check_json_content_type(request)?;

                    input = #{parser}(bytes.as_ref(), input)?;

                }

                """,

                *codegenScope,

                "parser" to parser,