IDEX-1265: return default Saas subscription on get subscription of accou...

...nt if there is no other subscriptions
@evoevodin @skabashnyuk @mshaposhnik 
i think <code>links = Collections.emptyList();</code> it is not the best way to do it right!
You can just declare new list in method start

``` java

private SubscriptionDescriptor toDescriptor(...) {
    final List<Link> links = new LinkedList<>();
    ...
}
```
so you will get rid of not needed <strike>unnecessary</strike>  <code>else</code> statement