fix: sql.NullString mistaken as custom struct

[CL-Jeremy]

Tested with this concise official example with connection setup changed to PostgreSQL:

bun/example/fixture/main.go

Lines 34 to 55 in 6c83b05

	func main() {
	ctx := context.Background()
	sqldb, err := sql.Open(sqliteshim.ShimName, "file::memory:?cache=shared")
	if err != nil {
	panic(err)
	}
	sqldb.SetMaxOpenConns(1)
	db := bun.NewDB(sqldb, sqlitedialect.New())
	db.AddQueryHook(bundebug.NewQueryHook(bundebug.WithVerbose(true)))
	// Register models before loading fixtures.
	db.RegisterModel((*User)(nil), (*Org)(nil))
	// Automatically create tables.
	fixture := dbfixture.New(db, dbfixture.WithRecreateTables())
	// Load fixtures.
	if err := fixture.Load(ctx, os.DirFS("."), "fixture.yml"); err != nil {
	panic(err)
	}

Without this fix, sql.NullString gets treated as a custom struct and then mapped to JSONB, resulting in failed insertion operations after the table has been created.

(Also removed currently unused, extraneous types.)

[CL-Jeremy]

Of course, there could be more tests added to avoid this type of mistakes systematically. Just a bit unsure where to put such tests (for PGSQL specifically, or should there be a general, complicated test, e. g. loading fixtures, enabled for all backends?)