fix(field-metadata): handle missing relationType in relation converter

https://sonarly.com/issue/35153?type=bug

UpdateField mutation fails with "Cannot return null for non-nullable field Relation.type" when querying relation metadata for fields with null or incomplete settings.

Fix: ## Changes Made

Modified the `fromMorphOrRelationFlatFieldMetadataToRelationDto` converter function to safely handle missing `settings.relationType`:

1. **from-morph-or-relation-flat-field-metadata-to-relation-dto.util.ts**:
   - Added optional chaining to safely access `sourceFlatFieldMetadata.settings?.relationType`
   - Return `null` early if relationType is missing, preventing undefined from being assigned to the non-nullable `type` field
   - Changed from inline object return to explicit null check + return block

2. **resolve-morph-relations-from-flat-field-metadata.util.ts**:
   - Updated to handle potential null return from converter
   - Changed from direct return to storing result in variable, then conditionally wrapping in array
   - Uses `flatMap` pattern: returns `[relationDto]` if valid, empty array `[]` if null

## Why This Fix Works

The GraphQL schema defines `Relation.type` as non-nullable (RelationDTO), but the parent field `relation` on FieldMetadata is nullable. When the converter returns `null`, GraphQL correctly resolves the parent `relation` field as `null` instead of throwing "Cannot return null for non-nullable field".

This matches the existing pattern in the codebase where:
- `resolveRelationFromFlatFieldMetadata` already returns `RelationDTO | null`
- The dataloader already expects `RelationDTO | null` (dataloader.service.ts:157)
- 10+ other files in the codebase use `settings?.relationType` with optional chaining

## Root Cause Addressed

Legacy relation fields created before settings validation was enforced (pre-Oct 2025 refactor, pre-Mar 2026 SDK validation) may have null or incomplete settings. The fix gracefully handles these fields by returning null for the entire relation instead of attempting to construct a RelationDTO with undefined type, which violated the GraphQL schema constraint.

packages/twenty-server/src/engine/metadata-modules/flat-field-metadata/utils/from-morph-or-relation-flat-field-metadata-to-relation-dto.util.ts

@@ -15,18 +15,26 @@ export const fromMorphOrRelationFlatFieldMetadataToRelationDto = ({
   sourceFlatObjectMetadata,
   targetFlatFieldMetadata,
   targetFlatObjectMetadata,
-}: FromMorphOrRelationFlatFieldMetadataToRelationDtoArgs) => ({
-  type: sourceFlatFieldMetadata.settings.relationType,
-  sourceObjectMetadata: fromFlatObjectMetadataToObjectMetadataDto(
-    sourceFlatObjectMetadata,
-  ),
-  sourceFieldMetadata: fromFlatFieldMetadataToFieldMetadataDto(
-    sourceFlatFieldMetadata,
-  ),
-  targetObjectMetadata: fromFlatObjectMetadataToObjectMetadataDto(
-    targetFlatObjectMetadata,
-  ),
-  targetFieldMetadata: fromFlatFieldMetadataToFieldMetadataDto(
-    targetFlatFieldMetadata,
-  ),
-});
+}: FromMorphOrRelationFlatFieldMetadataToRelationDtoArgs) => {
+  const relationType = sourceFlatFieldMetadata.settings?.relationType;
+
+  if (!relationType) {
+    return null;
+  }
+
+  return {
+    type: relationType,
+    sourceObjectMetadata: fromFlatObjectMetadataToObjectMetadataDto(
+      sourceFlatObjectMetadata,
+    ),
+    sourceFieldMetadata: fromFlatFieldMetadataToFieldMetadataDto(
+      sourceFlatFieldMetadata,
+    ),
+    targetObjectMetadata: fromFlatObjectMetadataToObjectMetadataDto(
+      targetFlatObjectMetadata,
+    ),
+    targetFieldMetadata: fromFlatFieldMetadataToFieldMetadataDto(
+      targetFlatFieldMetadata,
+    ),
+  };
+};

packages/twenty-server/src/engine/metadata-modules/flat-field-metadata/utils/resolve-morph-relations-from-flat-field-metadata.util.ts

@@ -60,7 +60,7 @@ export const resolveMorphRelationsFromFlatFieldMetadata = ({
         namePlural: targetFlatObjectMetadata.namePlural,
       });
 
-    return fromMorphOrRelationFlatFieldMetadataToRelationDto({
+    const relationDto = fromMorphOrRelationFlatFieldMetadataToRelationDto({
       sourceFlatFieldMetadata: {
         ...sourceFlatFieldMetadata,
         name: morphNameFromMorphFieldMetadataName,
@@ -69,5 +69,7 @@ export const resolveMorphRelationsFromFlatFieldMetadata = ({
       targetFlatObjectMetadata,
       sourceFlatObjectMetadata,
     });
+
+    return relationDto ? [relationDto] : [];
   });
 };