I feel like this is probably quite an obvious one, but I'm feeling a bit rushed and rusty...
I have a file containing constituent fields and gift fields. The constituent fields are always populated, but the gift fields are not always, depending on whether they have made a gift. Is there anything I can do to tell it to only import the gift fields if they are populated? e.g. if the Gift Amount field is populated, then it can go ahead and import that and the other gift fields.
Otherwise I will have to split the file in two, which isn't the end of the world, but I would prefer a more elegant, built-in solution.