Supabase adapter tries to serialize nested objects even when null, causing errors with foreign keys

[KiddoV]

Describe the bug

When using OfflineFirstWithSupabaseModel with nested relationships, Brick generates a Supabase insert/update that includes nested objects even when they are null. This causes errors like:

Could not find the 'assoc' column of 'tables' in the schema cache
insert or update on table violates foreign key constraint

The intended behavior is to serialize only the foreign key column when the nested object is null.

Minimal reproducible example

@ConnectOfflineFirstWithSupabase(
  supabaseConfig: SupabaseSerializable(tableName: "model_a")
)
@MappableClass()
class ModelA extends OfflineFirstWithSupabaseModel with ModelAMappable {
  final String id;

  @Supabase(foreignKey: "model_a_child1_id_fkey")
  final Child1? child1;

  @Supabase(foreignKey: "model_a_child2_id_fkey")
  final Child2? child2;

  @Sqlite(ignore: true)
  String? get child1Id => child1?.id;

  @Sqlite(ignore: true)
  String? get child2Id => child2?.id;

  ModelA({
    required this.id,
    this.child1,
    this.child2,
  });
}

@MappableClass()
class Child1 extends OfflineFirstWithSupabaseModel with Child1Mappable {
  final String id;
  Child1({required this.id});
}

@MappableClass()
class Child2 extends OfflineFirstWithSupabaseModel with Child2Mappable {
  final String id;
  Child2({required this.id});
}

Current behavior

If child1 is not null and child2 is null, Brick still construct upsert query with

child2:model_a_child2_id_fkey(*)....

Expect behavior

Brick should ignore this query in the overall constructed Supabase query string.