fix mypy error

Author: dantownsend

piccolo/columns/base.py

@@ -372,11 +372,11 @@ def _validate_default(
         ):
             self._validated = True
             return True
-        else:
-            raise ValueError(
-                f"The default {default} isn't one of the permitted types - "
-                f"{allowed_types}"
-            )
+
+        raise ValueError(
+            f"The default {default} isn't one of the permitted types - "
+            f"{allowed_types}"
+        )
 
     def _validate_choices(
         self, choices: t.Type[Enum], allowed_type: t.Type[t.Any]

tests/apps/migrations/auto/integration/test_migrations.py

@@ -152,7 +152,7 @@ def test_uuid_column(self):
                     UUID(default=uuid.uuid4),
                     UUID(default=uuid_default),
                     UUID(default=UUID4()),
-                    UUID(null=True),
+                    UUID(null=True, default=None),
                     UUID(null=False),
                     UUID(index=True),
                     UUID(index=False),