fix: the enum Value method returns correct type (#1996)

The Value method must returns nil or the following types:

* int64
* float64
* bool
* []byte
* string
* time.Time

Closes: #1880

Author: mcdoker18

examples/batch/postgresql/models.go

@@ -107,7 +107,7 @@ func (ns Null{{.Name}}) Value() (driver.Value, error) {
 	if !ns.Valid {
 		return nil, nil
 	}
-	return ns.{{.Name}}, nil
+	return string(ns.{{.Name}}), nil
 }