[Answered ]-Django INNER JOIN using foreign key with WHERE clause

You can exclude() the empty first_name values:

Friendship.objects.exclude(head_user__first_name='')

Note that if the first_name is NULL (not an empty string), you would need to use __isnull:

Friendship.objects.exclude(head_user__first_name__isnull=True)

And, yeah, you can chain excludes if you do want to exclude both NULL and empty values:

Friendship.objects.exclude(head_user__first_name__isnull=True) \
                  .exclude(head_user__first_name='')