fix: allow JSONField to be blank (#580)

* Fix new JSONField validation.

**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.

Co-authored-by: Arjuna Del Toso <arjuna@confirm.com>
Co-authored-by: Michal Čihař <michal@cihar.com>

social_django/migrations/0016_alter_usersocialauth_extra_data.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.25 on 2024-05-28 19:28
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("social_django", "0015_rename_extra_data_new_usersocialauth_extra_data"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="usersocialauth",
+            name="extra_data",
+            field=models.JSONField(blank=True, default=dict),
+        ),
+    ]

social_django/models.py

@@ -43,7 +43,7 @@ class AbstractUserSocialAuth(models.Model, DjangoUserMixin):
     )
     provider = models.CharField(max_length=32)
     uid = models.CharField(max_length=UID_LENGTH, db_index=True)
-    extra_data = models.JSONField(default=dict)
+    extra_data = models.JSONField(default=dict, blank=True)
     created = models.DateTimeField(auto_now_add=True)
     modified = models.DateTimeField(auto_now=True)
     objects = UserSocialAuthManager()