Preserve dynamic metadata for empty dependencies

[lin-hongkuan]

Summary of changes

Preserve dynamic core metadata for dependencies when project.dynamic = ['dependencies'] is supplied and the resolved dependency list is empty.

Setuptools already marked non-empty dynamic dependency lists as Dynamic: requires-dist because those lists are non-static values. Empty lists were skipped by the metadata writer's truthiness check, so an explicit install_requires=[] from setup.py produced no Dynamic field. This change records the corresponding core metadata field while applying pyproject.toml dynamic metadata, allowing empty dynamic dependency lists to stay dynamic without marking static dependencies = [] as dynamic.

Closes #5120

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
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Tests

• python -m pytest -q setuptools/tests/config/test_apply_pyprojecttoml.py setuptools/tests/config/test_pyprojecttoml_dynamic_deps.py setuptools/tests/test_core_metadata.py
• python -m ruff check setuptools/_core_metadata.py setuptools/config/_apply_pyprojecttoml.py setuptools/tests/config/test_apply_pyprojecttoml.py
• python -m ruff format --check setuptools/_core_metadata.py setuptools/config/_apply_pyprojecttoml.py setuptools/tests/config/test_apply_pyprojecttoml.py
• python -m mypy setuptools/_core_metadata.py setuptools/config/_apply_pyprojecttoml.py
• git diff --check

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