FIX Reject empty WMDP category values

[biefan]

Summary

• reject empty-string category values instead of treating them as "load all categories"
• add regression coverage to ensure invalid empty values fail fast without calling load_dataset

Problem

fetch_wmdp_dataset() documents None as the way to load all WMDP categories, and invalid category values are supposed to raise ValueError. The implementation currently uses if not category, which also treats "" as "load all categories".

As a result, an explicit but invalid empty-string category is silently ignored and the function loads all three subsets instead of failing fast.

Testing

• .venv/bin/pytest tests/unit/datasets/test_wmdp_dataset.py -q