**Description:**

Our setup (Django==3.2.25) has an automatic call to full_clean() on all Models in a pre-save signal.

After upgrading from 4.0.0 to 5.4.1 all our new Django Admins lost the ability to login.

We tracked it down to these calls here https://github.com/python-social-auth/social-app-django/blob/5.4.1/social_django/storage.py#L147, creation fails because `extra_data` is not passed to `create`, but the field is required not null and not blank and not empty dictionary.

Making it blank-able and null-able will solve the issue.

**TestPlan:**

Before the fix (any version between 5.3.0 and 5.4.1):

```
UserSocialAuth.objects.create(user=User.objects.first(), uid=1, provider="test")
[...]
ValidationError: {'extra_data': ['This field cannot be blank.']}
```

After the fix: the UserSocialAuth instance should be created correctly.

for more information, see https://pre-commit.ci

Co-authored-by: Michal Čihař <[email protected]>