`dolt diff` shows NULL values for virtual generated columns

Repro steps:

```
create table test(pk int primary key, c0 int generated always as (pk + 1));
insert into test(pk) values (1), (2);
```

`dolt diff` will produce the following output:

```
diff --dolt a/test b/test
added table
+CREATE TABLE `test` (
+ `pk` int NOT NULL,
+ `c0` int GENERATED ALWAYS AS ((`pk` + 1)),
+ PRIMARY KEY (`pk`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin;
+---+----+------+
| | pk | c0 |
+---+----+------+
| + | 1 | NULL |
| + | 2 | NULL |
+---+----+------+
```

This is likely because `c0` is virtual, so the tuples in the table don't actually contain a value for it. Displaying the correct values would require evaluating the stored expression for that column, which requires a SQL engine.

As for the correct behavior here, I'm leaning towards excluding virtual generated columns from `dolt diff` entirely, since they aren't actually part of the data in the diff.