Add applicationId to syncableEntity and fix syncApp deletion (#15170)

## Context
- All flatEntity should extend SyncableEntity
- SyncableEntity should now have applicationId and application relation
- Fix syncApp deletion, should now properly use migration v2 to delete
syncable entities

packages/twenty-server/src/engine/core-modules/applicationVariable/application-variable.service.ts

@@ -1,5 +1,6 @@
 import { InjectRepository } from '@nestjs/typeorm';
 
+import { isDefined } from 'twenty-shared/utils';
 import { In, Not, Repository } from 'typeorm';
 
 import { ApplicationVariable } from 'src/engine/core-modules/applicationVariable/application-variable.entity';
@@ -37,6 +38,10 @@ export class ApplicationVariableService {
     >;
     applicationId: string;
   }) {
+    if (!isDefined(env)) {
+      return;
+    }
+
     for (const [key, { value, description, isSecret }] of Object.entries(env)) {
       await this.applicationVariableRepository.upsert(
         {