sql: typechecking values clauses with tuples containing nulls is inconsistent with Postgres

The following expression fails to typecheck in CockroachDB, but succeeds in Postgres:

`VALUES((1, 2)), ((1, NULL))`

CockroachDB:
```
demo@127.0.0.1:26257/defaultdb> values((1, 2)), ((1, null));

ERROR: VALUES types tuple{int, unknown} and tuple{int, int} cannot be matched
SQLSTATE: 42804
```

Postgres:
```
jordan=> values((1, 2)), ((1, null));
column1
---------
(1,2)
(1,)
(2 rows)
```

Jira issue: CRDB-22279