fix: omit null computer safety checks

[sztoplover-bit]

Summary

• omit acknowledged_safety_checks from computer_call_output when it is None
• keep acknowledged safety checks unchanged when they are present
• add a regression assertion for the no-safety-check path

Testing

• .venv/bin/pytest tests/test_computer_action.py

Fixes #2741

[seratch]

There is already #2742; (the server side will eventually resolve this issue)