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

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -29,6 +29,7 @@ import java.nio.file.Files;
 import java.time.Duration;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Map;
 import java.util.function.Consumer;
 import java.util.stream.Stream;
 
@@ -1080,6 +1081,24 @@ class WebClientIntegrationTests {
 				.verify(Duration.ofSeconds(5));
 	}
 
+	@ParameterizedWebClientTest
+	void nullJsonResponseShouldBeReadAsEmpty(ClientHttpConnector connector) {
+		startServer(connector);
+
+		prepareResponse(response -> response
+				.setResponseCode(200)
+				.setHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
+				.setBody("null"));
+
+		Mono<Map> result = this.webClient.get()
+				.uri("/null")
+				.retrieve()
+				.bodyToMono(Map.class);
+
+		StepVerifier.create(result)
+				.verifyComplete();
+	}
+
 
 	private void prepareResponse(Consumer<MockResponse> consumer) {
 		MockResponse response = new MockResponse();