avoid storing empty collection

lib/storage.js

@@ -463,6 +463,13 @@ Storage.prototype.storeAddress = function(address, cb) {
 Storage.prototype.storeAddressAndWallet = function(wallet, addresses, cb) {
   var self = this;
 
+  function saveAddresses(addresses, cb) {
+    if (_.isEmpty(addresses)) return cb();
+    self.db.collection(collections.ADDRESSES).insert(addresses, {
+      w: 1
+    }, cb);
+  };
+
   var addresses = [].concat(addresses);
   if (addresses.length == 0) return cb();
 
@@ -484,9 +491,7 @@ Storage.prototype.storeAddressAndWallet = function(wallet, addresses, cb) {
     if (newAddresses.length < addresses.length) {
       log.warn('Attempted to store already existing addresses on wallet ' + wallet.id);
     }
-    self.db.collection(collections.ADDRESSES).insert(newAddresses, {
-      w: 1
-    }, function(err) {
+    saveAddresses(newAddresses, function(err) {
       if (err) return cb(err);
       self.storeWallet(wallet, cb);
     });