AD2CP conversion fails with NumPy 2.5

[LOCEANlloydizard]

Updating to NumPy 2.5 exposed an issue in one AD2CP conversion test (test_convert[8282021A1_avgd.ad2cp]). NaT values are passed through xarray's CF datetime decoder, which now raises during decoding. Investigating a fix to preserve NaT values while avoiding unnecessary CF datetime decoding.