fix: remove unnecessary 'or None' from extensions assignment

Author: jerome3o-anthropic

src/mcp/client/session.py

@@ -182,7 +182,7 @@ async def initialize(self) -> types.InitializeResult:
                             experimental=None,
                             roots=roots,
                             tasks=self._task_handlers.build_capability(),
-                            extensions=self._capability_extensions or None,
+                            extensions=self._capability_extensions,
                         ),
                         client_info=self._client_info,
                     ),