[Fix] Forbid default value removal for non-nullable select field (#16465)

Permanently fixes https://github.com/twentyhq/private-issues/issues/389

On a non-nullable select field, default value should not be removable,
otherwise users won't be able to create new records from the interface.
This PR enforces this in FE and BE.


https://github.com/user-attachments/assets/1dfa2bcc-b9df-4a00-9915-679d36ec1b25

packages/twenty-server/src/engine/workspace-manager/workspace-migration-v2/workspace-migration-builder-v2/validators/services/flat-field-metadata-validator.service.ts

@@ -158,6 +158,17 @@ export class FlatFieldMetadataValidatorService {
       });
     }
 
+    if (
+      flatFieldMetadataToValidate.isNullable === false &&
+      flatFieldMetadataToValidate.defaultValue === null
+    ) {
+      validationResult.errors.push({
+        code: FieldMetadataExceptionCode.INVALID_FIELD_INPUT,
+        message: 'Default value cannot be null for non-nullable fields',
+        userFriendlyMessage: msg`Default value cannot be null.`,
+      });
+    }
+
     const fieldMetadataTypeValidationErrors =
       this.flatFieldMetadataTypeValidatorService.validateFlatFieldMetadataTypeSpecificities(
         {