Merge doesn't validate not-null constraints for keyless tables

Repro:
```sql
CREATE TABLE aTable (aColumn INT NULL, bColumn INT NULL);
INSERT INTO aTable VALUES (1, 1);
CALL dolt_commit('-Am', 'add tables');
CALL dolt_checkout('-b', 'side');
INSERT INTO aTable VALUES (2,NULL);
CALL dolt_commit('-am', 'add side data');

CALL dolt_checkout('main');
ALTER TABLE aTable modify column bColumn INT NOT NULL;
CALL dolt_commit('-am', 'add main data');
CALL dolt_checkout('side');
SET @@dolt_force_transaction_commit=1;

-- This should return a response indicating a constraint violation, but instead it creates a new
call dolt_merge('main');

-- Looking at the table data shows the NULLs are in the table, even though the column does have the `NOT NULL ` constraint
select * from aTable;
show create table aTable;
```