Error on inserting empty byte array into JSON column using doltpy

For a table defined like this:

```sql
CREATE TABLE test (
id integer NOT NULL,
json_id json DEFAULT '{}' NOT NULL
)
```

Inserting into this table with `write_rows` fails if `json_id` is an empty byte array. Error:

`column name 'json_id' is non-nullable but attempted to set a value of null`