sqlite3 imports values as text and there does not seem to be a way to make it treat empty values as nulls.
However, you can update the tables yourself after import, setting empty strings to nulls, like
UPDATE t SET a1=NULL WHERE a1='';
Repeat for each column.
You can also create a trigger for such updates:
CREATE TRIGGER trig_a1 AFTER INSERT ON t WHEN new.a1='' BEGIN
UPDATE t SET a1=NULL WHERE rowid=new.rowid;
END;