Idiomatic Django conditional filter queryset
It's fine.
As this function doesn't have a default value for parameters,
you would have to use a dictionary if you want to make it more DRY.
I'd do something like:

kwargs = {
    'pk': pk,
    'user': user_pk
}
if not kwargs['user']:
    del kwargs['user']
queryset = Comment.objects.filter(**kwargs)