Reject null environment with a validation error

Address review feedback: raise ValidationError instead of passing
null through, since the nullable environment column is a historical
data layer artefact and not intended API behaviour.

Author: wavebyrd

api/features/serializers.py

@@ -598,7 +598,9 @@ def validate_feature(self, feature):  # type: ignore[no-untyped-def]
 
     def validate_environment(self, environment):  # type: ignore[no-untyped-def]
         if environment is None:
-            return environment
+            raise serializers.ValidationError(
+                "This field may not be null."
+            )
         if self.instance and self.instance.environment_id != environment.id:  # type: ignore[union-attr]
             raise serializers.ValidationError(
                 "Cannot change the environment of a feature state"

api/tests/unit/features/test_unit_features_serializers.py

@@ -14,7 +14,7 @@
 from features.serializers import FeatureStateSerializerBasic
 
 
-def test_feature_state_serializer_basic__null_environment_with_context__is_valid(  # type: ignore[no-untyped-def]
+def test_feature_state_serializer_basic__null_environment__returns_validation_error(  # type: ignore[no-untyped-def]
     feature, environment
 ):
     # Given
@@ -33,8 +33,9 @@ def test_feature_state_serializer_basic__null_environment_with_context__is_valid
     # When
     is_valid = serializer.is_valid()
 
-    # Then - should not raise AttributeError on environment.id
-    assert is_valid
+    # Then - should reject null environment, not raise AttributeError
+    assert not is_valid
+    assert "environment" in serializer.errors
 
 
 @pytest.mark.parametrize(