Replace incorrect "null" with None

aleksis/apps/hjelp/migrations/0004_unique_constraints.py

@@ -44,7 +44,7 @@ class Migration(migrations.Migration):
         ),
         migrations.AddConstraint(
             model_name='issuecategory',
-            constraint=models.UniqueConstraint(fields=('site_id', 'name'), condition=models.Q(parent='null'), name='unique_category_name_per_site_without_parent'),
+            constraint=models.UniqueConstraint(fields=('site_id', 'name'), condition=models.Q(parent=None), name='unique_category_name_per_site_without_parent'),
         ),
         migrations.AddConstraint(
             model_name='issuecategory',

aleksis/apps/hjelp/models.py

@@ -103,7 +103,7 @@ class IssueCategory(ExtensibleModel):
         constraints = [
             models.UniqueConstraint(
                 fields=["site_id", "name"],
-                condition=models.Q(parent="null"),
+                condition=models.Q(parent=None),
                 name="unique_category_name_per_site_without_parent",
             ),
             models.UniqueConstraint(