fix: use 'is not None' for related_request_id to handle id=0

When a JSON-RPC request uses id=0, notifications sent during tool
execution were not routed to the correct request stream because the
condition 'if related_request_id' evaluates to False for integer 0.

This caused notifications to be sent to the GET stream instead of the
POST stream for the specific request, making them invisible to clients
that don't have a GET SSE connection open (e.g. stateless mode).

Fix: change 'if related_request_id' to 'if related_request_id is not
None' in send_notification(), consistent with similar checks elsewhere
in the codebase (e.g. streamable_http.py line 984, 993, 997).

Fixes #1218

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/shared/session.py

@@ -303,7 +303,7 @@ async def send_notification(
         )
         session_message = SessionMessage(
             message=jsonrpc_notification,
-            metadata=ServerMessageMetadata(related_request_id=related_request_id) if related_request_id else None,
+            metadata=ServerMessageMetadata(related_request_id=related_request_id) if related_request_id is not None else None,
         )
         await self._write_stream.send(session_message)