[Minor] [Core] Don't NPE on closeQuietly(null)

Author: Aaron Davidson <[email protected]>

Closes #3166 from aarondav/closeQuietlyer and squashes the following commits:

78096b5 [Aaron Davidson] Don't NPE on closeQuietly(null)

Author: aarondav

network/common/src/main/java/org/apache/spark/network/util/JavaUtils.java

@@ -44,7 +44,9 @@ public class JavaUtils {
   /** Closes the given object, ignoring IOExceptions. */
   public static void closeQuietly(Closeable closeable) {
     try {
-      closeable.close();
+      if (closeable != null) {
+        closeable.close();
+      }
     } catch (IOException e) {
       logger.error("IOException should not have been thrown.", e);
     }