fix: set creative_id nullable in placement creation

src/usecase/purchase/create_placements.py

@@ -90,17 +90,20 @@ async def create_placements(
             or (details.creative_id if details and details.creative_id else None)
             or input.creative_id
         )
-        if not creative_id:
-            raise domain.CreativeNotFound()
+
+        # Если creative_id передан, ищем и валидируем креатив
+        creative: domain.Creative | None = None
+        if creative_id:
             creative = creatives_by_id.get(creative_id)
             if not creative:
                 creative = await self.database.get_creative(workspace_id, creative_id)
                 if not creative or creative.project_id != project.id:
                     raise domain.CreativeNotFound(creative_id)
                 creatives_by_id[creative_id] = creative
+
         placement = domain.Placement(
             project_id=project.id,
-            creative_id=creative.id,
+            creative_id=creative.id if creative else None,
             channel_id=channel.id,
             invite_link=None,
             invite_link_type=invite_link_type,