[3.11] GH-88050: fix race in closing subprocess pipe in asyncio (GH-97951) (#97978)

Check for None when iterating over `self._pipes.values()`.
(cherry picked from commit e2e6b95c0342247ed1a761b6e149ac579a8722dd)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>