null tolerant DocumentPath hasher

- DocumentPath parent can be null

Change-Id: If0daf8f05db47f532e406c9879ff94ce72693791
diff --git a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/PartitionedAsyncDocumentTree.java b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/PartitionedAsyncDocumentTree.java
index 3c53e30..2d17282 100644
--- a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/PartitionedAsyncDocumentTree.java
+++ b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/PartitionedAsyncDocumentTree.java
@@ -110,7 +110,7 @@
         // TODO: This operation is not atomic
         return partition(path.parent()).get(path).thenCompose(parentValue -> {
             if (parentValue == null) {
-                return Tools.exceptionalFuture(new NoSuchDocumentPathException(path.parent().toString()));
+                return Tools.exceptionalFuture(new NoSuchDocumentPathException(String.valueOf(path.parent())));
             } else {
                 return partition(path).createRecursive(path, value);
             }