Allow None on view_on_site (#2419)

This makes it consistent with `get_view_on_site_url` signature where it is used.

django-stubs/contrib/admin/options.pyi

@@ -139,7 +139,7 @@ class BaseModelAdmin(Generic[_ModelT]):
     def has_view_or_change_permission(self, request: HttpRequest, obj: _ModelT | None = ...) -> bool: ...
     def has_module_permission(self, request: HttpRequest) -> bool: ...
     @property
-    def view_on_site(self) -> Callable[[_ModelT], str] | bool: ...
+    def view_on_site(self) -> Callable[[_ModelT], str | None] | bool: ...
 
 _ModelAdmin = TypeVar("_ModelAdmin", bound=ModelAdmin[Any])
 _ActionCallable: TypeAlias = Callable[[_ModelAdmin, HttpRequest, QuerySet[_ModelT]], HttpResponseBase | None]