fix(aci): Add support for an empty conditionGroup in the API for the Error detector (#112641)

# Description
Fixes:
https://linear.app/getsentry/issue/ISWF-2357/saving-error-monitor-after-creating-alert-fails

<img width="1080" height="394" alt="Screenshot 2026-04-09 at 4 18 21 PM"
src="https://github.com/user-attachments/assets/6e5d899e-5aef-486c-ac19-da6bebfb620d"
/>

The issue seems like that the validator for ErrorDetectors was throwing
because it has an empty data condition group; this should be okay and
makes sense in our UI to provide as a default.

Rather than updating the UI to not send the field, this PR will allow
the `{}` for the condition group.

Author: saponifi3d

src/sentry/workflow_engine/endpoints/validators/base/detector.py

@@ -222,7 +222,7 @@ def update(self, instance: Detector, validated_data: dict[str, Any]) -> Detector
                     validated_data.pop("owner")
                 )
 
-            if "condition_group" in validated_data:
+            if "condition_group" in validated_data and validated_data.get("condition_group"):
                 condition_group = validated_data.pop("condition_group")
                 data_conditions: list[DataConditionType] = condition_group.get("conditions")
 

src/sentry/workflow_engine/endpoints/validators/error_detector.py

@@ -16,7 +16,14 @@
 class ErrorDetectorValidator(BaseDetectorTypeValidator):
     data_source_required = False
 
+    # TODO - Update the Error Detector to store grouping rules in the DataCondition
+    condition_group = serializers.DictField(
+        required=False,
+        allow_null=True,
+    )  # type: ignore[assignment]  # Intentionally overrides nested serializer to accept empty dicts
+
     fingerprinting_rules = serializers.CharField(required=False, allow_blank=True, allow_null=True)
+
     resolve_age = EmptyIntegerField(
         required=False,
         allow_null=True,
@@ -31,11 +38,12 @@ def validate_type(self, value: str) -> builtins.type[GroupType]:
         return type
 
     def validate_condition_group(self, value: Any) -> Any:
-        if value is not None:
+        if value:
             raise serializers.ValidationError(
                 "Condition group is not supported for error detectors"
             )
-        return value
+
+        return None
 
     def validate_name(self, value: Any) -> str:
         # if name is different from existing, raise an error

tests/sentry/workflow_engine/detectors/test_error_detector.py

@@ -82,6 +82,14 @@ def test_invalid_condition_group(self) -> None:
             )
         ]
 
+    def test_valid_condition_group(self) -> None:
+        data = {
+            **self.valid_data,
+            "condition_group": {},
+        }
+        validator = ErrorDetectorValidator(data=data, context=self.context)
+        assert validator.is_valid()
+
     def test_update_existing_with_valid_data(self) -> None:
         user = self.create_user()
         self.create_member(organization=self.project.organization, user=user)