When importing attributes which have a ‘Yes/No’ Data Type, Importacular by default will interpret a blank field in the data source as a ‘No’ and therefore add the Attribute with a value of 'No' to Raiser's Edge.

However, if you want to ensure the Attributes with a value of 'No' are not imported at all, there is a workaround as per below in this UK Tax Payer/Gift Aid example (we are adding an attribute of UK Tax Payer to those who have answered ‘Yes’ to gift aid):

Map the Category field to your source (in this case, GiftAid?) and write a conditional add on the field using the Data Transformation feature, so the ‘Yes’ will be transformed to UK Tax Payer. The second line in the Data Transformation uses the 'Catch All' feature to ensure that anything other than a 'Yes' is left blank, disabling the 'From Source' cell for that row. This will prevent the attribute from being added at all when the incoming value is blank. 

Here is the mapping: 

Here is the custom change within the field settings:

Here is the Data Review Screen before importing: