Fix return type of dbm.whichdb (#10989)

It is `None` if the given file doesn't exist or isn't a database.

stdlib/dbm/__init__.pyi

@@ -90,5 +90,5 @@ class _error(Exception): ...
 
 error: tuple[type[_error], type[OSError]]
 
-def whichdb(filename: str) -> str: ...
+def whichdb(filename: str) -> str | None: ...
 def open(file: str, flag: _TFlags = "r", mode: int = 0o666) -> _Database: ...