Null equivalence - Fix (#16050)

@charlesBochet 
In workflow codebase, NULL (instead of empty object) is expected on
workflow version object step field, when a new workflow is created for
example.
(packages/twenty-front/src/modules/workflow/workflow-diagram/utils/generateWorkflowDiagram.ts
- 42)
This case is not isolated and it creates many issues.

We decided to format NULL value to equivalent (empty string for text
field, empty object for raw_json) but it seems it complicates the dev x.

To unlock @Devessier I prefer revert the logic, the time we discuss how
to solve this cases.

packages/twenty-server/src/modules/workflow/workflow-trigger/jobs/workflow-trigger.job.ts

@@ -1,8 +1,8 @@
 import { Scope } from '@nestjs/common';
 
-import { isDefined } from 'class-validator';
 import isEmpty from 'lodash.isempty';
 import { FieldActorSource } from 'twenty-shared/types';
+import { isDefined } from 'twenty-shared/utils';
 
 import { InjectMessageQueue } from 'src/engine/core-modules/message-queue/decorators/message-queue.decorator';
 import { Process } from 'src/engine/core-modules/message-queue/decorators/process.decorator';