return null instead of undefined, if not provider is set

lib/providermanager.js

@@ -70,7 +70,7 @@ ProviderManager.prototype.send = function(data) {
 
     if (this.provider === undefined) {
         console.error('provider is not set');
-        return undefined;
+        return null; 
     }
 
     //TODO: handle error here?