Jackson2JsonDecoder tolerates null literal

Fixes #22042

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -38,6 +38,7 @@ import org.springframework.core.codec.AbstractDecoderTestCase;
 import org.springframework.core.codec.CodecException;
 import org.springframework.core.codec.DecodingException;
 import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.http.MediaType;
 import org.springframework.http.codec.Pojo;
 import org.springframework.util.MimeType;
 
@@ -174,6 +175,14 @@ public class Jackson2JsonDecoderTests extends AbstractDecoderTestCase<Jackson2Js
 				.verifyError(DecodingException.class));
 	}
 
+	@Test // #22042
+	public void decodeWithNullLiteral() {
+		Flux<Object> result = this.decoder.decode(Flux.concat(stringBuffer("null")),
+				ResolvableType.forType(Pojo.class), MediaType.APPLICATION_JSON, Collections.emptyMap());
+
+		StepVerifier.create(result).expectComplete().verify();
+	}
+
 	@Test
 	public void noDefaultConstructor() {
 		Flux<DataBuffer> input =