The following logic for ImportFilter::filterRow works (at least for my scenario in which I don't use empty strings)...

    public Object[] filterRow(Object[] row) throws SQLException, IOException {

    Object[] lclReturn = new Object[row.length];

    for (int i = 0; i < row.length; i++) {

        Object lclObject = row[i];

        if (lclObject != null) {

        if (!"".equals(lclObject.toString().trim())) {
            lclReturn[i] = lclObject;
        } else {
            lclReturn[i] = null;
        }

        } else {
        lclReturn[i] = null;
        }

    }

    return lclReturn;
    }