AutoMigrate fails with sql: expected 0 arguments, got 1 (#131)

* AutoMigrate fails with `sql: expected 0 arguments, got 1` when `Statement.Vars` is null

* Update migrator.go

Co-authored-by: Jinzhu <wosmvp@gmail.com>

migrator.go

@@ -528,7 +528,7 @@ func (m Migrator) GetRows(currentSchema interface{}, table interface{}) (*sql.Ro
 
 	return m.DB.Session(&gorm.Session{}).Table(name).Limit(1).Scopes(func(d *gorm.DB) *gorm.DB {
 		// use simple protocol
-		if !m.DB.PrepareStmt {
+		if !m.DB.PrepareStmt && len(d.Statement.Vars) != 0 {
 			d.Statement.Vars = append(d.Statement.Vars, pgx.QuerySimpleProtocol(true))
 		}
 		return d