Fix tuple deformation with virtual generated NOT NULL columns

Hi,

While testing "Optimize tuple deformation”, I found a bug:
```
evantest=# create table t (a int not null,
evantest(# g int generated always as (a+1) virtual not null,
evantest(# b int not null);
CREATE TABLE
evantest=# insert into t (a, b) values (10, 20);
INSERT 0 1
evantest=# select a, g, b from t;
 a  | g  | b
----+----+---
 10 | 11 | 0
(1 row)
```

Here, b was inserted as 20, but select only returned 0.

I think the problem is in finding the first non-guaranteed attribute where virtual generated attributes are not considered:
```
	for (int i = 0; i < tupdesc->natts; i++)
	{
		CompactAttribute *cattr = TupleDescCompactAttr(tupdesc, i);

		/*
		 * Find the highest attnum which is guaranteed to exist in all tuples
		 * in the table.  We currently only pay attention to byval attributes
		 * to allow additional optimizations during tuple deformation.
		 */
		if (firstNonGuaranteedAttr == tupdesc->natts &&
			(cattr->attnullability != ATTNULLABLE_VALID || !cattr->attbyval ||
			 cattr->atthasmissing || cattr->attisdropped || cattr->attlen <= 0))
			firstNonGuaranteedAttr = i;
```

To fix this, we should consider virtual generated attributes as non-guaranteed. The tricky part is that cattr->attgenerated is only a boolean and cannot distinguish virtual generated from stored. So we have to further check TupleDescAttr(tupdesc, i)->attgenerated. In the patch, I changed the check as follows:
```
		if (firstNonGuaranteedAttr == tupdesc->natts &&
			(cattr->attnullability != ATTNULLABLE_VALID || !cattr->attbyval ||
			 cattr->atthasmissing || cattr->attisdropped || cattr->attlen <= 0 ||
			 (cattr->attgenerated &&
			  TupleDescAttr(tupdesc, i)->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)))
			firstNonGuaranteedAttr = i;
```

This way, we only check TupleDescAttr(tupdesc, i)->attgenerated when needed.

See the attached patch for details. I also added a regression test case to cover this fix. With the fix, select now returns correct values:
```
evantest=# select a, g, b from t;
 a  | g  | b
----+----+----
 10 | 11 | 20
(1 row)
```

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/