Fixing ScrollReaderTest compilation (#1887)

Author: masseyke

mr/src/test/java/org/elasticsearch/hadoop/serialization/ScrollReaderTest.java

@@ -53,6 +53,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;