dynamic filter_spec: no variable selected should mean no filtering, and not filter out all data

NEST/teal.devel/issues/716

currently:


Despite `SEX` being selected in the `select_spec` there is no faceting because `filter_spec` is not selected. Currently, if `filter_spec` is not selected then it filters out all data.

At the very least, I think that if the variable of the `filter_spec` is not selected, then it should behave as if the `filter_spec` was simply not provided, i.e. no filtering.

We should ask, are there cases where the user would want to filter out ALL data?
If not, then even in the case where the variable is not empty but the value is empty, it should also mean no filtering, and not filter out all like it does currently:


Provenance:
```
Creator: junlue
```