create_tile_layers should handle null typename

In a GeoNode istance some layers have the `typename` attribute set to null, so the line
```py
   r = requests.get(f"{baseurl}gwc/rest/layers/{layer.typename}.xml", auth=HTTPBasicAuth(user, passwd))
``` 
will likely fail with a 404.
The `create_tile_layers ` command should fallback to the `resource.alternate` value in case `dataset.typename` is null.