common api - null equivalence (#15926)

closes https://github.com/twentyhq/core-team-issues/issues/1629

To do before requesting review : 
- filter update

Migration to come in an other PR


Strat : 
1/  Null transformation

- [x] Transform NULL equivalent value to NULL in field validation in
common api - pre-query - with feature flag
- [ ] Same logic in ORM (Not done, complex to handle feature flag here)
- [x] Transform NULL value to equivalent in data formatting in ORM -
post-query

2/ Migration (in other PR) for fieldMetadata not nullable with default
defaultValue (empty string, ...)

- [ ] Remove NOT NULL db constraint
- [ ] Update record value to NULL
- [ ] Update field metadata : isNullable:true
- [ ] Update uniqueIndex whereClause (also for standard uniqueIndex) 
- [ ] Activate feature flag

3/ Update metadata creation

- [x] No more default default value
- [x] Update standard field nullability
- [x] Remove index default whereClause for standard field

4/ Update filter

- [x] When filtering on NULL or empty string, be sure all records are
returned (the one with NULL + the one with "")

5/ Test

- [ ] Strat. to do

packages/twenty-server/src/engine/twenty-orm/entity-manager/workspace-entity-manager.spec.ts

@@ -140,6 +140,7 @@ describe('WorkspaceEntityManager', () => {
         IS_WORKFLOW_RUN_STOPPAGE_ENABLED: false,
         IS_DASHBOARD_V2_ENABLED: false,
         IS_GLOBAL_WORKSPACE_DATASOURCE_ENABLED: false,
+        IS_NULL_EQUIVALENCE_ENABLED: false,
       },
       eventEmitterService: {
         emitMutationEvent: jest.fn(),
@@ -167,6 +168,7 @@ describe('WorkspaceEntityManager', () => {
         IS_WORKFLOW_RUN_STOPPAGE_ENABLED: false,
         IS_DASHBOARD_V2_ENABLED: false,
         IS_GLOBAL_WORKSPACE_DATASOURCE_ENABLED: false,
+        IS_NULL_EQUIVALENCE_ENABLED: false,
       },
       permissionsPerRoleId: {},
     } as WorkspaceDataSource;