When you are importing data from a database, such as a CSV file, Monarch lets you replace empty cells with default values. For numeric fields, 0 is the usual default value, for character fields, "" (the empty string), and for dates, 1 January 1970. You can change these settings somewhere if you need.
It's worth choosing this option because null fields can be a pain later on. If you have a calculated field based on one of your database fields, a null value in the database field will produce (usually) a null value in the calculated field, for example.
You can get around it by including:
in your calculated fields, or by using a lookup field with rules to replace null inputs.