[site] add(user)sitepackages(known_paths) shouldn't be None (#15866)

Author: srittau

stdlib/site.pyi

@@ -21,8 +21,8 @@ if sys.version_info >= (3, 15):
         def process(self) -> None: ...
 
 def addsitedir(sitedir: str, known_paths: set[str] | None = None) -> None: ...
-def addsitepackages(known_paths: set[str] | None, prefixes: Iterable[str] | None = None) -> set[str] | None: ...  # undocumented
-def addusersitepackages(known_paths: set[str] | None) -> set[str] | None: ...  # undocumented
+def addsitepackages(known_paths: set[str], prefixes: Iterable[str] | None = None) -> set[str]: ...  # undocumented
+def addusersitepackages(known_paths: set[str]) -> set[str]: ...  # undocumented
 def check_enableusersite() -> bool | None: ...  # undocumented
 
 if sys.version_info >= (3, 13):