Allow for decode method to return null in AbstractJackson2Decoder

Prior to this commit, the decoder did not allow for a null value
returned from the Jackson object reader.

Closes: gh-23935

spring-web/src/main/java/org/springframework/http/codec/json/AbstractJackson2Decoder.java

@@ -130,7 +130,7 @@ public abstract class AbstractJackson2Decoder extends Jackson2CodecSupport imple
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		return DataBufferUtils.join(input, this.maxInMemorySize)
-				.map(dataBuffer -> decode(dataBuffer, elementType, mimeType, hints));
+				.flatMap(dataBuffer -> Mono.justOrEmpty(decode(dataBuffer, elementType, mimeType, hints)));
 	}
 
 	@Override