`VALUES` is never considered non-nullable

[neilconway]

Describe the bug

Expressions from VALUES are always considered nullable, which (1) inhibits some optimization opportunities, at least in theory (2) is inconsistent with other similar situations where we do nullability analysis; see example.

To Reproduce

I think the behavior here is inconsistent:

> DESCRIBE SELECT 1 FROM VALUES (5);
+-------------+-----------+-------------+
| column_name | data_type | is_nullable |
+-------------+-----------+-------------+
| Int64(1)    | Int64     | NO          |
+-------------+-----------+-------------+
1 row(s) fetched.
Elapsed 0.002 seconds.

> DESCRIBE VALUES (5);
+-------------+-----------+-------------+
| column_name | data_type | is_nullable |
+-------------+-----------+-------------+
| column1     | Int64     | YES         |
+-------------+-----------+-------------+
1 row(s) fetched.
Elapsed 0.002 seconds.

Expected behavior

No response

Additional context

No response

[neilconway]

take