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.