core: add @nullable annotation

Subchannel can be nullable, add annotation for consitency
diff --git a/core/src/main/java/io/grpc/LoadBalancer.java b/core/src/main/java/io/grpc/LoadBalancer.java
index 1db9d36..d46d50a 100644
--- a/core/src/main/java/io/grpc/LoadBalancer.java
+++ b/core/src/main/java/io/grpc/LoadBalancer.java
@@ -199,7 +199,7 @@
     // subchannel being null and error being OK means RPC needs to wait
     private final Status status;
 
-    private PickResult(Subchannel subchannel, Status status) {
+    private PickResult(@Nullable Subchannel subchannel, Status status) {
       this.subchannel = subchannel;
       this.status = Preconditions.checkNotNull(status, "status");
     }