Fix xarray compatibility issues and improve empty dataset handling

[femiomitusa]

• Fix datetime extraction from xarray DataArray using .item() method (xarray datetime accessor now returns DataArrays, not scalars)
• Replace deprecated Dataset.dims[] with Dataset.sizes[] for dimension lengths to avoid FutureWarning in xarray >= 2024.10
• Add missing datetime import in mimic_lma.py
• Handle empty datasets gracefully in get_lutevents() by returning early with an empty lutevent_id dimension
• Use assert_allclose instead of assert_equal for floating point comparisons in test_event_lut.py to account for precision limits