Exception during test collection results in ZeroDivisionError

In short, this:
`pytest --splits 3 --group 1`

results in this
```
File "/home/user/.venv/lib/python3.8/site-packages/pytest_split/plugin.py", line 114, in _calculate_suite_start_and_end_idx
avg_duration_per_test = sum(stored_durations.values()) / len(stored_durations)
ZeroDivisionError: division by zero
```

if there's e.g. some `ImportError` during test collection.

The ZeroDivisionError hides the original exception.

使用 `pytest --splits 3 --group 1` 重现失败，然后检查 `pytest_split/plugin.py` 中的 `_calculate_suite_start_and_end_idx`，尤其是第 114 行。确认 collection 时的 `ImportError` 正被替换为 `ZeroDivisionError`；完成标准是原始的 collection 异常仍然可见，而不是被替换。

由索引模型根据 Issue 内容生成。