Comply with RET501

arkindex/project/serializer_fields.py

@@ -304,4 +304,4 @@ class NullField(serializers.CharField):
     def to_internal_value(self, data):
         if data is not None:
             self.fail("invalid")
-        return None
+        return

ruff.toml

@@ -8,5 +8,7 @@ quote-style = "double"
 select = ["Q0", "F", "W", "E",
     # request-without-timeout
     "S113",
+    # flake8-return: unnecessary-return-none
+    "RET501",
 ]
 ignore = ["E501"]