Support empty body in Jackson2JsonDecoder.decodeToMono

Prior to this commit, extracting an HTTP response with an empty body
and no Content-Type header using the WebClient would:

* trigger the use of the Jackson2JsonDecoder
* throw a NoSuchElementException because of the use of `Flux.single()`

This commit changes this behavior to `Flux.singleOrEmpty()` to avoid
throwing exceptions for empty Flux instances.

Issue: SPR-14582

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonDecoderTests.java

@@ -105,6 +105,18 @@ public class Jackson2JsonDecoderTests extends AbstractDataBufferAllocatingTestCa
 				});
 	}
 
+	@Test
+	public void decodeEmptyBodyToMono() throws Exception {
+		Flux<DataBuffer> source = Flux.empty();
+		ResolvableType elementType = ResolvableType.forClass(Pojo.class);
+		Mono<Object> flux = new Jackson2JsonDecoder().decodeToMono(source, elementType, null);
+
+		TestSubscriber.subscribe(flux)
+				.assertNoError()
+				.assertComplete()
+				.assertValueCount(0);
+	}
+
 
 	void handle(List<Pojo> list) {
 	}