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/workspace-manager/workspace-migration-v2/workspace-migration-builder-v2/validators/services/flat-index-metadata-validator.service.ts

@@ -8,7 +8,6 @@ import {
 } from 'twenty-shared/types';
 import { isDefined } from 'twenty-shared/utils';
 
-import { isValidUniqueFieldDefaultValueCombination } from 'src/engine/metadata-modules/field-metadata/utils/is-valid-unique-input.util';
 import { FlatEntityMapsExceptionCode } from 'src/engine/metadata-modules/flat-entity/exceptions/flat-entity-maps.exception';
 import { findFlatEntityByIdInFlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/utils/find-flat-entity-by-id-in-flat-entity-maps.util';
 import { isCompositeFlatFieldMetadata } from 'src/engine/metadata-modules/flat-field-metadata/utils/is-composite-flat-field-metadata.util';
@@ -147,11 +146,7 @@ export class FlatIndexValidatorService {
           if (flatIndexToValidate.isUnique) {
             if (
               isDefined(relatedFlatField.defaultValue) &&
-              !isValidUniqueFieldDefaultValueCombination({
-                defaultValue: relatedFlatField.defaultValue,
-                isUnique: relatedFlatField.isUnique ?? false,
-                type: relatedFlatField.type,
-              })
+              relatedFlatField.isUnique
             ) {
               const fieldName = relatedFlatField.name;
               const fieldType = relatedFlatField.type;