Aggregate Columns: silent fail on sparse

**What's wrong?**

Aggregate Columns silently fails on sparse data, i.e. returns all nan values (when it shouldn't).

**How can we reproduce the problem?**

Corpus - BoW - Aggregate Columns

**What's your environment?**

- Operating system: OSX
- Orange version: 3.37.0.dev
- How you installed Orange: git

Reproduce the issue through the Corpus → BoW → Aggregate Columns workflow using sparse data. Trace the Aggregate Columns widget and its tests or data-processing entry points to find why the result becomes all NaN. Done means sparse input produces the expected aggregate values instead of silently returning all NaN.

Written by the indexing model from the issue text.