One approach is to first import the data into a staging table.
Then, run a query that copies data from the staging table into the final table, using the case construct or the NULLIF function in the select subquery, to insert a null in the target field when the source field is an empty string. (or, more accurately, when the staging table source field contains just the two quotation characters "")
Select field1, NULLIF(field2, '""') AS field2, NULLIF(field3, ''"") as field3 FROM staging_table
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