GithubApp.slug is None due to no `_completeIfNotSet` call

I noticed that when I do the following code, it will print out `None`. I saw that the slug attribute doesn't use `_completeIfNotSet` before returning the value.:

```python
g = Github(jwt="jwt_token_here")
app = g.get_app()
print(f"slug: {app.slug}")
```

https://github.com/PyGithub/PyGithub/blob/master/github/GithubApp.py#L107-L112