Fix empty body writing in EncoderHttpMessageWriter

Prior to this commit, an bug introduced in SPR-16949 prevented
`Mono.empty` bodies from being written to the response.

This commit ensures that empty bodies still trigger the writing to the
response and does not hang the processing of the exchange.

Issue: SPR-17220
Cherry-picked from: 280da61d5c

spring-web/src/main/java/org/springframework/http/codec/EncoderHttpMessageWriter.java

@@ -47,6 +47,7 @@ import org.springframework.util.Assert;
  * @author Arjen Poutsma
  * @author Sebastien Deleuze
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.0
  */
 public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
@@ -107,9 +108,10 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 			HttpHeaders headers = message.getHeaders();
 			if (headers.getContentLength() < 0 && !headers.containsKey(HttpHeaders.TRANSFER_ENCODING)) {
 				return Mono.from(body)
-						.flatMap(dataBuffer -> {
-							headers.setContentLength(dataBuffer.readableByteCount());
-							return message.writeWith(Mono.just(dataBuffer));
+						.defaultIfEmpty(message.bufferFactory().wrap(new byte[0]))
+						.flatMap(buffer -> {
+							headers.setContentLength(buffer.readableByteCount());
+							return message.writeWith(Mono.just(buffer));
 						});
 			}
 		}