Typer-style interface throws an assertion when no callback is present on a subgroup.

```python
  from django_typer.management import Typer
  
  app = Typer()
  
  grp = Typer()
  app.add_typer(grp, name="grp")
  
  @grp.command()
  def cmd():
      print("grp:cmd")
```

The above fails because no callback is registered for grp, but it should work.