bug: emtpy list has the meaning of subscribed to everything

collector.py:147

```
sdata = subscription.collector_data
name = 'selected_collectors'
if name in sdata and sdata[name] and self not in sdata[name]:
return [], now
```

if sdata[name] is a empty list then then the above code makes the assumption that the subscription is subscribed to everything - this does seem to be rather over-assuming since an empty list is more likely to mean explicitly not subscribed to anything. Suggest changing it to

```
sdata = subscription.collector_data
name = 'selected_collectors'
if name in sdata and self not in sdata[name]:
return [], now
```

Start at collector.py:147 and inspect how subscription.collector_data and selected_collectors are interpreted. Change the empty-list behavior so it does not imply subscription to every collector, then verify that an empty selected_collectors list returns no collectors while the field’s absence preserves the existing behavior.

Written by the indexing model from the issue text.