Fix relation field unknown target object (#13129)

Fixes https://github.com/twentyhq/twenty/issues/12867

Issue:
when you have a variable `toto` which is: `Record<string, MyType>` and
you do toto['xxx'], this will be typed as `MyType` instead of `MyType |
undefined`

Solutions:
- activate `noUncheckedIndexedAccess` check in tsconfig, this is the
preferred solution but will take time to get there (this raises 600+
errors)
- use a Map: cf https://github.com/twentyhq/twenty/pull/13125/files
- set the type to Partial<Record<string, MyType>>. Drawback is that when
you do Object.values(toto), you'll get `Array<MyType | undefined>`.
Hence why we have to filter these behind


<img width="1512" alt="image"
src="https://github.com/user-attachments/assets/d0a0bfed-c441-4e53-84c2-2da98ccbcf50"
/>

packages/twenty-server/src/engine/api/rest/core/interfaces/rest-api-base.handler.ts

@@ -171,6 +171,12 @@ export abstract class RestApiBaseHandler {
               objectMetadata.objectMetadataMaps.byId[
                 field.relationTargetObjectMetadataId
               ];
+
+            if (!isDefined(relationTargetObjectMetadata)) {
+              throw new BadRequestException(
+                `Object metadata relation target not found for relation creation payload`,
+              );
+            }
             const depth2Relations = this.getRelations({
               objectMetadata: {
                 objectMetadataMaps: objectMetadata.objectMetadataMaps,

packages/twenty-server/src/engine/api/rest/core/query-builder/utils/map-field-metadata-to-graphql-query.utils.ts

@@ -1,4 +1,5 @@
 import { FieldMetadataType } from 'twenty-shared/types';
+import { isDefined } from 'twenty-shared/utils';
 
 import { FieldMetadataInterface } from 'src/engine/metadata-modules/field-metadata/interfaces/field-metadata.interface';
 import { RelationType } from 'src/engine/metadata-modules/field-metadata/interfaces/relation-type.interface';
@@ -58,6 +59,10 @@ export const mapFieldMetadataToGraphqlQuery = (
     const relationMetadataItem =
       objectMetadataMaps.byId[targetObjectMetadataId];
 
+    if (!isDefined(relationMetadataItem)) {
+      return '';
+    }
+
     return `${field.name}
     {
       id