Incorrect result of isNullable for constant values

`isNullable` for `SELECT 1` returns 1 (`columnNullable`) instead of 0 (`columnNoNulls`) or 2 (`columnNullableUnknown`).

```java
//DEPS com.amazon.redshift:redshift-jdbc42:2.1.0.34

import java.sql.*;

public class RedshiftDemo {
private static final String DB_URL = System.getenv("REDSHIFT_URL");
private static final String DB_USER = System.getenv("REDSHIFT_USER");
private static final String DB_PASSWORD = System.getenv("REDSHIFT_PASSWORD");

public static void main(String[] args) throws Exception {
try (Connection conn = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT 1, 'hi'")) {

System.out.println("Nullable[1]:" + rs.getMetaData().isNullable(1));
System.out.println("Nullable[2]:" + rs.getMetaData().isNullable(2));
rs.next();
System.out.printf("Query result: %s %s%n", rs.getInt(1), rs.getObject(2));
}
}
}
```

```
Nullable[1]:1
Nullable[2]:1
Query result: 1 hi
```

Run the provided Java JDBC reproduction against Redshift and start at ResultSet.getMetaData().isNullable(). Trace how metadata for the constant columns in SELECT 1, 'hi' is classified. Done means the constant columns return columnNoNulls or columnNullableUnknown instead of columnNullable, with regression coverage for this query.

Written by the indexing model from the issue text.