Fix MultiBacktest when some runs make no trades

[BerqiaMouad]

Summary

• keep the existing public stats series for zero-trade runs instead of replacing it with None
• add regression coverage for all-zero and mixed zero/non-zero MultiBacktest runs

Why

Backtest.run() already returns meaningful public stats when a strategy makes zero trades.
Dropping those results to None makes MultiBacktest.run() either raise when a batch mixes Series and None, or return a one-row frame of None values.

Fixes #1344.

Validation

• python -m unittest -v backtesting.test._test.TestLib.test_MultiBacktest backtesting.test._test.TestLib.test_MultiBacktest_keeps_zero_trade_runs
• flake8 backtesting setup.py

Notes

• On my local macOS / Python 3.11.14 / pandas 3.0.2 setup, python -m backtesting.test still hits the pre-existing test_FractionalBacktest failure (ValueError: output array is read-only).