Initialize from_order_func record buffers before exposing them to callbacks

[Whning0513]

Fixes #858.

Summary

• initialize empty order and log record buffers with stable sentinel values
• initialize empty last-position records with stable sentinel values before callback exposure
• add a regression test covering Portfolio.from_order_func(..., use_numba=False) before any order is filled

Testing

• python -m pytest tests\test_portfolio.py -k "initialized_records" -q
• python -m pytest tests\test_portfolio.py -k "test_pre_sim_ctx or test_post_sim_ctx" -q

[polakowo]

Thanks! Merged.