Handle empty columns in dictionary check

[AtomicGlance]

Summary

• Skip empty table columns before inspecting their first value in check_table_values_for_dict.
• Add a regression test covering an empty DynamicTable column.

Why

DynamicTable and EventsTable can define columns before any rows are added. The check previously indexed column.data[0] unconditionally, causing an IndexError during inspection instead of returning no finding.

Fixes #712.

Validation

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1, PYTHONPATH=src: 377 passed, 30 skipped
• ruff check src/nwbinspector/checks/_tables.py tests/unit_tests/test_tables.py: passed
• Black was not available in the local environment.

[AtomicGlance]

The empty-column dictionary check is covered by a regression test, and the available CI checks are green. The PR is ready for review.