`sql.NullX` used incorrectly in postgres queries

[Gilthans]

Describe the bug
When using a null sql.NullInt32 in a query using postgres dialect, the created query uses WHERE x = NULL, instead of the correct WHERE x is NULL.

To Reproduce

package main

import (
	"database/sql"
	"fmt"
	"github.com/doug-martin/goqu/v9"
)

func main() {
	sql, _, _ := goqu.Dialect("postgres").From(goqu.T("table")).Select("Id").Where(goqu.Ex{"Val": sql.NullInt32{}}).ToSQL()
	fmt.Println(sql)
	sql, _, _ = goqu.Dialect("postgres").From(goqu.T("table")).Select("Id").Where(goqu.Ex{"Val": nil}).ToSQL()
	fmt.Println(sql)
}

=>
SELECT "Id" FROM "table" WHERE ("Val" = NULL) // Wrong
SELECT "Id" FROM "table" WHERE ("Val" IS NULL) // Right

Expected behavior
When using sql.NullX as a null value, expect to use correctly

Dialect:

• postgres
• mysql
• sqlite3

[nitaysol]

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