Hey @sr112233!
Passing an empty value for a particular cell (i.e. ,,) should read as null for "date" columns, however, for "enum" or other text columns the value is indistinguishable from the empty string and gets read as the latter. It doesn't look like Retool has a way to specifically set a null value identifier at the moment.
I can file it as a feature request with the dev team - would you mind expanding a bit on why it's important for your use case to use null? Is it general best practice or something more specific?