Fix NPE in StoragePartition

Change-Id: Ibd71260381d79304243ae7448ff4b2155483dd5a
diff --git a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/StoragePartition.java b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/StoragePartition.java
index d8fb580..41cdf7c 100644
--- a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/StoragePartition.java
+++ b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/StoragePartition.java
@@ -78,7 +78,7 @@
 
     @Override
     public CompletableFuture<Void> open() {
-        return openServer().thenAccept(s -> server = Optional.of(s))
+        return openServer().thenAccept(s -> server = Optional.ofNullable(s))
                            .thenCompose(v-> openClient())
                            .thenAccept(v -> isOpened.set(true))
                            .thenApply(v -> null);