Default for generated column not working as expected

SetUp:
`create table t1 (i int primary key, j int generated always as (i + 10));`
`insert into t1 (i, j) values (1, default);`

Query: `select * from t1`
Expected: `[]sql.Row{{1, 11}}`
Actual: `[]sql.Row{{1, nil}}`

Test case skipped and tagged with issue link in dolthub/go-mysql-server#3705

Start by reproducing the CREATE TABLE, INSERT, and SELECT statements from the issue, then inspect the skipped test tagged with dolthub/go-mysql-server#3705. Trace generated-column handling for an explicit DEFAULT value and confirm that the query returns {{1, 11}} instead of a nil value.

Written by the indexing model from the issue text.