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/modules/workflow/workflow-builder/workflow-schema/utils/generate-fake-form-response.ts

@@ -30,10 +30,11 @@ export const generateFakeFormResponse = async ({
             formFieldMetadata?.settings?.objectName,
           );
 
-        if (!objectMetadataItemWithFieldsMaps)
+        if (!isDefined(objectMetadataItemWithFieldsMaps)) {
           throw new Error(
             `Object metadata not found for object name ${formFieldMetadata?.settings?.objectName}`,
           );
+        }
 
         return {
           [formFieldMetadata.name]: {

packages/twenty-server/src/modules/workflow/workflow-builder/workflow-schema/utils/generate-object-record-fields.ts

@@ -43,6 +43,10 @@ export const generateObjectRecordFields = ({
             field.relationTargetObjectMetadataId
           ];
 
+        if (!isDefined(relationTargetObjectMetadata)) {
+          return acc;
+        }
+
         acc[field.name] = {
           isLeaf: false,
           icon: field.icon,