Add @Nullable annotation to decode method taking DataBuffer

Javadoc indicates the returned value can be null.

spring-core/src/main/java/org/springframework/core/codec/Decoder.java

 /*
- * Copyright 2002-2016 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.
@@ -88,7 +88,7 @@ public interface Decoder<T> {
 	 * @return the decoded value, possibly {@code null}
 	 * @since 5.2
 	 */
-	@SuppressWarnings("ConstantConditions")
+	@Nullable
 	default T decode(DataBuffer buffer, ResolvableType targetType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) throws DecodingException {