V2: Query API incompatibility around missing time windows

#### Describe the bug
A clear and concise description of what the bug is.

When using the v2 mode, the API behaves directly, it seems to treat the time window as required:

```
$ curl -H "content-type: application/json" -d '{"label_names":["__profile_type__"]}' "http://v1/querier.v1.QuerierService/Series"
{"labelsSet":[{"labels":[...]}]}

$ curl -H "content-type: application/json" -d '{"label_names":["__profile_type__"]}' "http://v2/querier.v1.QuerierService/Series"
{"code":"invalid_argument","message":"missing time range in the query"}%

```

#### Expected behavior

Follow the same API behaviour, with and without v2