lab_compare classification export: '<' not supported between str and NoneType when sorting value tuples

🤖 Written by Claude

The lab-compare classification export builds `(value, source)` tuples where some sources are `None`, then `sorted()` raises `TypeError: '<' not supported between instances of 'str' and 'NoneType'` for records with partial evidence keys.

Seen on sharianttest. Fix = coerce `None` in the sort key.

Location: `classification/views/exports/classification_export_formatter_lab_compare.py:39` (`list_of`)

**Rollbar:**
- https://app.rollbar.com/a/jimmy.andrews/fix/item/VariantGrid/7152

_(Filed from a 6-month Rollbar triage.)_