Data Loader behavior with Bulk API enabled using 'Insert null values'

Enabling the Bulk API in Data Loader allows you to load or delete a large number of records faster than using the default SOAP-based API. However, there are some differences in behavior in Data Loader when you enable the Bulk API. One important difference is that it allows you to execute a hard delete if you have the permission and license. See Configuring Data Loader.

The following settings are not available on the Settings | Settings page in Data Loader when the Use Bulk API option is selected:

Insert null values
This option enables Data Loader to insert blank mapped values as null values during data operations when the Bulk API is disabled. Empty field values are ignored when you update records using the Bulk API. To set a field value to null when the Use Bulk API option is selected, use a field value of #N/A​.

If you want to use import wizard for null values below article will be helpful also

Insert null value to a field with the Data Import Wizard
 

Allow field truncation
This option directs Data Loader to truncate data for certain field types when the Bulk API is disabled. A load operation fails for the row if a value is specified that is too large for the field when the Use Bulk API option is selected.

Data Loader Behavior with Bulk API Enabled