CSV import does not handle empty field / NULL correctly

[neilotoole]

Describe the bug

CSV empty fields should be handled as NULL, not empty string.

To Reproduce

$ cat address.csv | sq '.data | .[:5]'
address_id  address               address2  district  city_id  postal_code  phone  last_update
1           47 MySakila Drive                         300      NULL                2020-02-15T06:59:28Z
2           28 MySQL Boulevard                        576      NULL                2020-02-15T06:59:28Z
3           23 Workhaven Lane                         300      NULL                2020-02-15T06:59:28Z
4           1411 Lillydale Drive                      576      NULL                2020-02-15T06:59:28Z
5           1913 Hanoi Way                            463      35200               2020-02-15T06:59:28Z

In JSON:

{"address_id": 1, "address": "47 MySakila Drive", "address2": "", "district": "", "city_id": 300, "postal_code": null, "phone": "", "last_update": "2020-02-15T06:59:28Z"}
{"address_id": 2, "address": "28 MySQL Boulevard", "address2": "", "district": "", "city_id": 576, "postal_code": null, "phone": "", "last_update": "2020-02-15T06:59:28Z"}
{"address_id": 3, "address": "23 Workhaven Lane", "address2": "", "district": "", "city_id": 300, "postal_code": null, "phone": "", "last_update": "2020-02-15T06:59:28Z"}
{"address_id": 4, "address": "1411 Lillydale Drive", "address2": "", "district": "", "city_id": 576, "postal_code": null, "phone": "", "last_update": "2020-02-15T06:59:28Z"}
{"address_id": 5, "address": "1913 Hanoi Way", "address2": "", "district": "", "city_id": 463, "postal_code": 35200, "phone": "", "last_update": "2020-02-15T06:59:28Z"}

Expected behavior
I expected the address2 and district fields to be null in the example above.

[neilotoole]

Fixed.