fix: Author the column default an upsert writes for a null foreign key (#164)

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

test/serverpod_offline_sync_test_server/lib/src/generated/protocol.dart

@@ -2123,6 +2123,23 @@ class Protocol extends _is.DatabaseSerializationManager {
           isUnique: true,
           isPrimary: false,
         ),
+        _isp.IndexDefinition(
+          indexName: 'unique_set_default_child_name',
+          tableSpace: null,
+          elements: [
+            _isp.IndexElementDefinition(
+              type: _isp.IndexElementDefinitionType.column,
+              definition: 'spaceId',
+            ),
+            _isp.IndexElementDefinition(
+              type: _isp.IndexElementDefinitionType.column,
+              definition: 'name',
+            ),
+          ],
+          type: 'btree',
+          isUnique: true,
+          isPrimary: false,
+        ),
       ],
       managed: true,
     ),

test/serverpod_offline_sync_test_server/lib/src/models/unique_set_default_child.spy.yaml

@@ -6,3 +6,7 @@ fields:
   # Keep the database default without replacing explicit nulls on model reads.
   parentId: UuidValue?, defaultPersist='550e8400-e29b-41d4-a716-446655440000', unique
   parent: Town?, relation(field=parentId, onDelete=SetDefault, deferred)
+indexes:
+  unique_set_default_child_name:
+    fields: spaceId, name
+    unique: true