HADOOP-16218. Findbugs warning of null param to non-nullable method in Configuration with Guava update. (#655)

Change-Id: I461e518ce9a4730b91a8138ad55b39e9a4b0a4b8

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/conf/Configuration.java

@@ -69,6 +69,7 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
+import javax.annotation.Nullable;
 import javax.xml.parsers.DocumentBuilderFactory;
 import javax.xml.parsers.ParserConfigurationException;
 import javax.xml.stream.XMLInputFactory;
@@ -3512,7 +3513,7 @@ public class Configuration implements Iterable<Map.Entry<String,String>>,
    * </ul>
    * @param out the writer to write to.
    */
-  public void writeXml(String propertyName, Writer out)
+  public void writeXml(@Nullable String propertyName, Writer out)
       throws IOException, IllegalArgumentException {
     Document doc = asXmlDocument(propertyName);
 
@@ -3534,7 +3535,7 @@ public class Configuration implements Iterable<Map.Entry<String,String>>,
   /**
    * Return the XML DOM corresponding to this Configuration.
    */
-  private synchronized Document asXmlDocument(String propertyName)
+  private synchronized Document asXmlDocument(@Nullable String propertyName)
       throws IOException, IllegalArgumentException {
     Document doc;
     try {