Table import can allow NULLs in the primary key

[Hydrocharged]

When using dolt table import -c, we currently do not follow the rule that primary keys may not contain any NULLs. This will put the table into an invalid state. It can be reproduced by:

$ dolt init
Successfully initialized dolt data repository.

$ cat <<DELIM > null-pk.csv
pk,v1
"a",1
,2
DELIM

$ dolt table import -c --pk=pk test null-pk.csv
Rows Processed: 2, Additions: 2, Modifications: 0, Had No Effect: 0
Import completed successfully.

$ dolt sql -q "select * from test"
+------+----+
| pk   | v1 |
+------+----+
| a    | 1  |
| NULL | 2  |
+------+----+

$ dolt sql -q "update test set pk = 'b' where pk is null"
column <pk> received nil but is non-nullable