invalid empirical model is queried

[nandaloo]

When you work with variables that are not observed, i.e. that are not backed by data, then invalid queries to the references empirical model are issued. This crashes the frontend.

We solve this as follows

• [x] Make sure that the base model and base model table of the empirical model can be derived. This is achieved by filtering out any modelling clauses that are invalid, i.e. filter all Fields and FieldUsages of variables that are not in the empirical model.
• [x] Make sure that any query that is executed always returns data table of results. In case of an error, this table may be empty, but it must exists. This is because the code depends on the existence of that result table.
• [ ] fix new issue, see comment below...

[jong42]

The similar problem appears when working with variables that are not modeled, i.e. for which no samples are generated. Then invalid queries to the model are issued when activating the 'model marginals' facet

[nandaloo]

I see. I'll check it out.