Add non-empty constraint for UserSocialAuth uid

Any empty uids are surely erroneous, and better to have integrity errors
than having users start sharing accounts when logging in.

social_django/migrations/0016_add_not_empty_string_constraint_to_uid_usersocialauth.py

@@ -0,0 +1,19 @@
+# Generated by Django 5.0.3 on 2024-03-21 16:01
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("social_django", "0015_rename_extra_data_new_usersocialauth_extra_data"),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name="UserSocialAuth",
+            constraint=models.CheckConstraint(
+                check=models.Q(("uid", ""), _negated=True),
+                name="user_social_auth_uid_required",
+            ),
+        ),
+    ]

social_django/models.py

@@ -80,6 +80,11 @@ class Meta:
         app_label = "social_django"
         unique_together = ("provider", "uid")
         db_table = "social_auth_usersocialauth"
+        constraints = [
+            models.CheckConstraint(
+                check=~models.Q(uid=""), name="user_social_auth_uid_required"
+            ),
+        ]
 
 
 class Nonce(models.Model, DjangoNonceMixin):