Improve empty body check

Fixes spring-projectsgh-22265

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/client/MessageBodyClientHttpResponseWrapper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 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.
@@ -79,8 +79,13 @@ public boolean hasMessageBody() throws IOException {
 	 * @return {@code true} if the response has a zero-length message body, {@code false} otherwise
 	 * @throws IOException in case of I/O errors
 	 */
+	@SuppressWarnings("ConstantConditions")
 	public boolean hasEmptyMessageBody() throws IOException {
 		InputStream body = this.response.getBody();
+		// Per contract body shouldn't be null, but check anyway..
+		if (body == null) {
+			return true;
+		}
 		if (body.markSupported()) {
 			body.mark(1);
 			if (body.read() == -1) {

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

@@ -175,7 +175,7 @@ public void invalidData() {
 				.verifyError(DecodingException.class));
 	}
 
-	@Test // #22042
+	@Test // gh-22042
 	public void decodeWithNullLiteral() {
 		Flux<Object> result = this.decoder.decode(Flux.concat(stringBuffer("null")),
 				ResolvableType.forType(Pojo.class), MediaType.APPLICATION_JSON, Collections.emptyMap());

spring-web/src/test/java/org/springframework/web/client/HttpMessageConverterExtractorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 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.
@@ -113,6 +113,20 @@ public void emptyMessageBody() throws IOException {
 		assertNull(result);
 	}
 
+	@Test // gh-22265
+	@SuppressWarnings("unchecked")
+	public void nullMessageBody() throws IOException {
+		HttpMessageConverter<String> converter = mock(HttpMessageConverter.class);
+		HttpHeaders responseHeaders = new HttpHeaders();
+		extractor = new HttpMessageConverterExtractor<>(String.class, createConverterList(converter));
+		given(response.getRawStatusCode()).willReturn(HttpStatus.OK.value());
+		given(response.getHeaders()).willReturn(responseHeaders);
+		given(response.getBody()).willReturn(null);
+
+		Object result = extractor.extractData(response);
+		assertNull(result);
+	}
+
 	@Test
 	@SuppressWarnings("unchecked")
 	public void normal() throws IOException {