Importing NULL values errors even when default is provided

Importing into a field with a default value for nulls errors for null values.

```
An error occurred while moving data
cause: column name 'cl_sortkey_prefix' is non-nullable but attempted to set a value of null
A bad row was encountered inserting into table categorylinks (on line 9):
cl_from: 13305
cl_to: Linguistic_morphology
cl_sortkey: MORPHOPHONOLOGY
cl_sortkey_prefix:
cl_timestamp: 2024-03-12 02:10:51
cl_collation: uppercase
cl_type: page
```
#!/bin/bash

rm -rf tmp1
mkdir tmp1
cd tmp1
dolt init

dolt sql < test1.csv
cl_from,cl_to,cl_sortkey,cl_sortkey_prefix,cl_timestamp,cl_collation,cl_type
cl_from,cl_to,cl_sortkey,cl_sortkey_prefix,cl_timestamp,cl_collation,cl_type
13303,Communes_of_Tarn-et-Garonne,MONTAUBAN,,2024-03-12 02:10:49,uppercase,page
13303,Pages_with_broken_file_links,MONTAUBAN,,2024-03-12 02:10:49,uppercase,page
13303,Prefectures_in_France,MONTAUBAN,,2024-03-12 02:10:49,uppercase,page
13303,Quercy,MONTAUBAN,,2024-03-12 02:10:49,uppercase,page
13304,Pages_with_broken_file_links,RAIL TRANSPORT MODELLING,,2024-03-12 02:10:50,uppercase,page
13304,Rail_transport_modelling, RAIL TRANSPORT MODELLING, ,2024-03-12 02:10:50,uppercase,page
13305,Linguistic_morphology,MORPHOPHONOLOGY,,2024-03-12 02:10:51,uppercase,page
13305,Morphophonology, MORPHOPHONOLOGY, ,2024-03-12 02:10:51,uppercase,page
13305,Orthography,MORPHOPHONOLOGY,,2024-03-12 02:10:51,uppercase,page
13305,Phonology,MORPHOPHONOLOGY,,2024-03-12 02:10:51,uppercase,page
13306,Glass_applications,MIRROR,,2024-03-12 02:10:52,uppercase,page
EOF

dolt table import -u categorylinks test1.csv
```

Replacing nulls with empty strings `''` fixes:
```
cl_from,cl_to,cl_sortkey,cl_sortkey_prefix,cl_timestamp,cl_collation,cl_type
13303,Communes_of_Tarn-et-Garonne,MONTAUBAN,'',2024-03-12 02:10:49,uppercase,page
13303,Pages_with_broken_file_links,MONTAUBAN,'',2024-03-12 02:10:49,uppercase,page
13303,Prefectures_in_France,MONTAUBAN,'',2024-03-12 02:10:49,uppercase,page
13303,Quercy,MONTAUBAN,'',2024-03-12 02:10:49,uppercase,page
13304,Miniature_railways, RAIL TRANSPORT MODELLING,'',2024-03-12 02:10:50,uppercase,page
13304,Pages_with_broken_file_links,RAIL TRANSPORT MODELLING,'',2024-03-12 02:10:50,uppercase,page
13304,Rail_transport_modelling, RAIL TRANSPORT MODELLING,'',2024-03-12 02:10:50,uppercase,page
13305,Linguistic_morphology,MORPHOPHONOLOGY,'',2024-03-12 02:10:51,uppercase,page
13305,Morphophonology, MORPHOPHONOLOGY,'',2024-03-12 02:10:51,uppercase,page
13305,Orthography,MORPHOPHONOLOGY,'',2024-03-12 02:10:51,uppercase,page
13305,Phonology,MORPHOPHONOLOGY,'',2024-03-12 02:10:51,uppercase,page
13306,Glass_applications,MIRROR,'',2024-03-12 02:10:52,uppercase,page
```