I have the following `SQL` table:

```
create table dbo.[File]
(
  Id int identity not null
    constraint PK_File_Id primary key clustered (Id),
  MimeType nvarchar (400) not null,
  Name nvarchar (80) null,
  Slug nvarchar (80) null
)

```

How can I create a constraint that allows `Name` and `Slug` to be both `null` but never only one of them. So when `Name` or `Slug` is not `null` the other must also be not `null`.