PROBLEM
Updating a shapefile's numeric or text value with NULL shows up as zero or as an empty string in ArcMap.
Shapefiles do not support NULL values. DBase 7 supports NULL values; shapefiles are pre-dBase 7.
ArcView 3.x uses a masking technique to present certain values as NULL, although they are not true NULL values.
Get help from ArcGIS experts
Download the Esri Support App