Fix MultiBacktest mixed no-trade results

[elpolini]

Summary

Fixes #1344.
Closes #1358

MultiBacktest._mp_task_run() currently returns None for datasets whose strategy produced zero trades. When a MultiBacktest run mixes datasets with trades and datasets with no trades, MultiBacktest.run() then receives a mixed list of pd.Series and None, which can either raise TypeError: object of type 'NoneType' has no len() or produce nested/odd DataFrame values depending on ordering.

Backtest.run() already returns a complete stats series for zero-trade strategies, so this change keeps those stats instead of replacing them with None.

Changes

• Always return filtered stats from _mp_task_run(), including zero-trade stats.
• Add a regression test for mixed trade/no-trade datasets.
• Assert zero-trade datasets report # Trades == 0 and do not produce nested Series values.

Test plan

python -m unittest backtesting.test._test.TestLib.test_MultiBacktest_handles_mixed_no_trade_results
python -m unittest backtesting.test._test.TestLib.test_MultiBacktest
python -m unittest backtesting.test._test.TestBacktest.test_run backtesting.test._test.TestUtil.test_as_str
python -m compileall -q backtesting
flake8 backtesting setup.py

[kernc]

This is now green and lgtm! Thanks!