Mark SharedMemory.buf optional (#14723)

stdlib/multiprocessing/shared_memory.pyi

@@ -15,7 +15,7 @@ class SharedMemory:
         def __init__(self, name: str | None = None, create: bool = False, size: int = 0) -> None: ...
 
     @property
-    def buf(self) -> memoryview: ...
+    def buf(self) -> memoryview | None: ...
     @property
     def name(self) -> str: ...
     @property