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/metadata-modules/flat-field-metadata/utils/get-default-flat-field-metadata-from-create-field-input.util.ts

@@ -34,7 +34,6 @@ export const getDefaultFlatFieldMetadata = ({
     isCustom: true,
     isLabelSyncedWithName: createFieldInput.isLabelSyncedWithName ?? false,
     isNullable: generateNullable(
-      createFieldInput.type,
       createFieldInput.isNullable,
       createFieldInput.isRemoteCreation,
     ),