[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 e2e6b95)

Co-authored-by: Kumar Aditya <[email protected]>

Author: miss-islington

Lib/asyncio/base_subprocess.py

@@ -216,7 +216,9 @@ def _process_exited(self, returncode):
             self._proc.returncode = returncode
         self._call(self._protocol.process_exited)
         for p in self._pipes.values():
-            p.pipe.close()
+            if p is not None:
+                p.pipe.close()
+
         self._try_finish()
 
     async def _wait(self):