Introduce getOrEmpty(String) convenience method in HttpHeaders

This commit introduces a getOrEmpty(String) method in HttpHeaders that
returns an immutable, empty list if no values are present for the
specified header name. This is provided as a convenience over the
existing get(String) method which returns null in such cases.

Closes gh-22949

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -71,6 +71,7 @@ import org.springframework.util.StringUtils;
  * @author Brian Clozel
  * @author Juergen Hoeller
  * @author Josh Long
+ * @author Sam Brannen
  * @since 3.0
  */
 public class HttpHeaders implements MultiValueMap<String, String>, Serializable {
@@ -437,6 +438,17 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	}
 
 
+	/**
+	 * Get the list of header values for the given header name, if any.
+	 * @param headerName the header name
+	 * @return the list of header values, or an empty list
+	 * @since 5.2
+	 */
+	public List<String> getOrEmpty(Object headerName) {
+		List<String> values = get(headerName);
+		return (values != null ? values : Collections.emptyList());
+	}
+
 	/**
 	 * Set the list of acceptable {@linkplain MediaType media types},
 	 * as specified by the {@code Accept} header.