graph/db: only init extra byte if not nil

In preparation for having consistency with the structs created by the
SQLStore and the KVStore (so that they have the same behaviour when
tested by the unit tests), here we make sure not to init the
ExtraOpaqueData field of the LightningNode struct unless there are
actualy bytes to set.

graph/db/sql_store.go

@@ -494,10 +494,9 @@ func buildNode(ctx context.Context, db SQLQueries, dbNode *sqlc.Node) (
 	copy(pub[:], dbNode.PubKey)
 
 	node := &models.LightningNode{
-		PubKeyBytes:     pub,
-		Features:        lnwire.EmptyFeatureVector(),
-		LastUpdate:      time.Unix(0, 0),
-		ExtraOpaqueData: make([]byte, 0),
+		PubKeyBytes: pub,
+		Features:    lnwire.EmptyFeatureVector(),
+		LastUpdate:  time.Unix(0, 0),
 	}
 
 	if len(dbNode.Signature) == 0 {