Time fields cannot be set to empty in Sao
Time fields cannot be left empty, or set to an empty value in Sao.
If viewing a record with a None / null value in a Time field, then trying to move off the record, or close the tab, results in the time being set to "00:00:00" and a message indicating the the record has been modified and asking if you would like to save it. Also clearing the time from a time field, and saving the record results in the time being filled back in as "00:00:00", so it is not possible to set Time values to None from Sao.
It doesn't look like there is a problem with the Desktop client, as clearing the time from a Time field results in the time being left empty.