persistent: first example needs EmptyDataDecls

```
src/Main.hs:12:1:
    `Unique' has no constructors (-XEmptyDataDecls permits this)
    In the data type instance declaration for `Unique'
    In the instance declaration for `PersistEntity (BlogPostGeneric backend)'

src/Main.hs:12:1:
    `Unique' has no constructors (-XEmptyDataDecls permits this)
    In the data type instance declaration for `Unique'
    In the instance declaration for `PersistEntity (PersonGeneric backend)'
```