Same "null" value problem like in #99

I have same problem with "null" value like in https://github.com/go-gorm/datatypes/issues/99

But with JSONSlice.

I have column definition:
```
Customs     datatypes.JSONSlice[*mytypes.KeyValue]
```

and when create model:
```
Customs:     datatypes.NewJSONSlice(req.Msg.GetCustoms()), // return []*mytypes.KeyValue
```

when it's empty I have "null" value in column instead of real `NULL`