Why is the field set to blank instead of null and vice versa in ArcGIS Pro?

Answer

In ArcGIS Pro, after deleting the value of a field and pressing Enter in an attribute table, the value is displayed as <Null> or blank. While both results indicate empty values, there is a difference in both returns. Null values represent fields with no data whereas a blank field exists as a string with no characters. A further explanation is provided below to demonstrate why the field is set to blank or null.

Null values

A field can have a null value when the data type is set to numeric or the Allow NULL check box is enabled. It is not possible to have a blank value as an integer such as the number zero ('0') is also a value. For example, in an attribute table, when the value in the No. of Siblings field is deleted, the field is set to <Null> instead of blank, demonstrating the value does not exist.

To allow null values in a field when creating the field in the attribute table, set the data type to a numeric data type or check the Allow NULL check box. Refer to ArcGIS Pro: Create and manage fields or How To: Assign nulls to a field in ArcGIS Pro for instructions.

Note: 
In some instances, due to BUG-000145688, a field is set to blank even when the Allow NULL check box is enabled and the data type is set to numeric. In this case, upgrade ArcGIS Pro to the latest version.

Blank fields

A field can be blank when the data type is set to text or the Allow NULL check box is disabled. For example, when a user does not disclose their address and the field value is removed, the field is set to blank instead of <Null>.

To allow a blank field when creating the field in the attribute table, disable the Allow NULL check box or set the data type to text. Refer to ArcGIS Pro: Create and manage fields for instructions.