Subscription operations return wrong status codes when there are no pools.

[Eva Balycheva]

When pooling is enabled in Zaqar and there are no pools registered (and so no queue):
Subscription operations on non-existing queue:
1. create request returns 409 code response with no body which is bad.
2. list request returns 500 code response with body:
{
    "title": "Internal server error",
    "description": "NoneType object is not an iterator"
}
Which is very bad.
3. get request returns 200 code response with body 'null', which is bad.
4. delete request returns 204 code response with no body, which is okay.