_sort_results raises KeyError when every game in a bulk scrape fails #89

Latent bug (pre-2.2.0, not a regression)

The bulk scrapers (`get_games_range`, `get_games_team`, `get_games_conference`) end with `_sort_results`, which sorts the info frame by `['game_datetime', 'game_id']` (src/cbbpy/utils/cbbpy_utils.py:279). When game IDs exist but every `_get_game` call returns empty frames (e.g. ESPN outage, WAF blocking, all games 404), `pd.concat` of the empty `pd.DataFrame([])` results yields an empty frame with no columns, and `sort_values` raises `KeyError: 'game_datetime'` instead of returning three empty DataFrames.

Master had the same hole (it sorted by `game_day`/`game_time`), so this is not a 2.2.0 regression — but #83 was specifically about failure-path return types, and this is the remaining gap.

Fix

In `_sort_results`, skip sorting any frame that is empty (or lacks the sort columns), so the all-failures path returns `(empty, empty, empty)` like the no-games path does. Add an offline test where every game fetch fails.