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/test/integration/metadata/suites/field-metadata/__snapshots__/failing-field-metadata-relation-creation.integration-spec.ts.snap

@@ -60,7 +60,7 @@ exports[`Field metadata relation creation should fail relation when targetObject
       "exceptionEventId": "mocked-exception-id",
       "userFriendlyMessage": "An error occurred.",
     },
-    "message": "Cannot read properties of undefined (reading 'fieldsById')",
+    "message": "Object metadata relation target not found for relation creation payload",
   },
 ]
 `;