[federetyk]

Description

• Task.__init__ silently produced an empty task in two cases: when the requested languages matched no dataset, and when every selected dataset was skipped with DatasetConfigNotSupported during loading. Both cases now log a warning that names the task and explains why it is empty.
• For cross-lingual tasks the first case is easy to hit: a dataset is selected only when the requested languages cover both its query and target languages, so SearchQueryCandidateRanking(split="test", languages=["de"]) selects nothing. The warning states this and shows the required call shape (e.g. languages=["de", "en"] for a German-to-English dataset).
• Motivated by a RecSys 2026 reviewer report: the call above returned empty results with no indication why, and the reviewer concluded the cross-lingual CRec evaluation was not reproducible from the documented API.
• Behavior is otherwise unchanged. An empty task still evaluates to no results, which existing flows rely on (e.g. examples/run_paper_results.py expects MELSRanking to have zero datasets for languages=["en"]).
• Tests: new TestEmptyDatasetWarnings in tests/test_task_loading.py covers both warnings plus a no-warning control case. The tests use an autouse fixture that temporarily enables propagation on the workrb logger, since setup_logger sets propagate = False, which hides records from pytest's caplog.

Checklist

• Added new tests for new functionality
• Tested locally with example tasks
• Code follows project style guidelines
• Documentation updated
• No new warnings introduced
• If the rankings artifact schema changed: bumped SCHEMA_VERSION in workrb/rankings.py and updated SUPPORTED_SCHEMA_VERSIONS

[warreveys]

Thanks for the contribution!