Add test for empty subspaces forward pass

[aryamanarora]

Summary

• add an integration test that checks the intervened forward pass handles an empty subspaces list the same as omitting the parameter

Testing

• pytest tests/integration_tests/IntervenableBasicTestCase.py::IntervenableBasicTestCase::test_empty_subspaces_matches_default_behavior

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https://chatgpt.com/codex/tasks/task_e_68ed7e60b6e483248fc27103c9a50665

[aryamanarora]

nvm this is not needed