Make sure origin billing entity is not mandatory

ref #38

src/servala/core/migrations/0011_alter_organizationorigin_billing_entity.py

@@ -0,0 +1,27 @@
+# Generated by Django 5.2.7 on 2025-10-22 13:40
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("core", "0010_remove_invitation_unique_constraint"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="organizationorigin",
+            name="billing_entity",
+            field=models.ForeignKey(
+                blank=True,
+                help_text="If set, this billing entity will be used on new organizations with this origin.",
+                null=True,
+                on_delete=django.db.models.deletion.PROTECT,
+                related_name="origins",
+                to="core.billingentity",
+                verbose_name="Billing entity",
+            ),
+        ),
+    ]

src/servala/core/models/organization.py

@@ -387,7 +387,7 @@ class OrganizationOrigin(ServalaModelMixin, models.Model):
         help_text=_(
             "If set, this billing entity will be used on new organizations with this origin."
         ),
-        null=True,
+        null=True, blank=True,
     )
     limit_cloudproviders = models.ManyToManyField(
         to="CloudProvider",