Skip missing temperature adjustment values

[akmhatey-ai]

Summary

• Preserve missing temperature sensor values when applying configured temperature adjustments.
• This prevents startup from crashing when a BMS reports sensors 1/2 but leaves sensor 3/4 or MOS temperature as None.
• Added a small regression test for the adjustment helper and a changelog entry.

Fixes #457.

Validation

• uv run --with pytest --with pyserial pytest tests/test_dbushelper_temperature.py -q -> 5 passed
• uv run --with pytest --with pyserial pytest tests/test_dbushelper_temperature.py tests/test_cached_dbus_proxy.py tests/test_get_bus.py -q -> 36 passed
• uv run --with pytest --with pyserial pytest --ignore=tests/bms -q -> 54 passed
• uv run --with black black --check dbus-serialbattery/dbushelper.py tests/test_dbushelper_temperature.py -> passed
• uv run --with flake8 flake8 dbus-serialbattery/dbushelper.py tests/test_dbushelper_temperature.py -> passed
• git diff --cached --check -> passed
• gitleaks protect --staged --redact --verbose -> no leaks found

I also tried uv run --with pytest --with pyserial pytest -q; on Windows it stops during collection because tests/bms/test_lltjbd_up16s.py imports the Unix-only termios module.

[mr-manuel]

Thanks!