null-safe DocPath ancentry tests

Change-Id: I36709bee94b10e516427adb892d1caeabea1b9aa
diff --git a/core/api/src/test/java/org/onosproject/store/service/DocumentPathTest.java b/core/api/src/test/java/org/onosproject/store/service/DocumentPathTest.java
index 19c34bd..431f58f 100644
--- a/core/api/src/test/java/org/onosproject/store/service/DocumentPathTest.java
+++ b/core/api/src/test/java/org/onosproject/store/service/DocumentPathTest.java
@@ -60,6 +60,9 @@
         assertTrue(path3.isDescendentOf(path3));
         assertTrue(path3.isDescendentOf(path1));
         assertFalse(path3.isDescendentOf(path2));
+
+        assertFalse(path.isDescendentOf(null));
+        assertFalse(path.isAncestorOf(null));
     }
 
     @Rule