Editing a row with NA strings turns them into ""

[calpaterson]

Description

NA strings get clobbered on save from web ui

Steps to reproduce

1. Open any row with <NA> in text, eg https://csvbase.com/calpaterson/palmer-penguins/rows/4
2. Make no changes
3. Click save

Expected result

sex is left as <NA>

Actual result

sex is now ""

Additional details

Need a way for this form to indicate that text fields are <NA>