fix annotation for select_jinja_autoescape (#5808)

Author: davidism

src/flask/sansio/app.py

@@ -533,7 +533,7 @@ def create_global_jinja_loader(self) -> DispatchingJinjaLoader:
         """
         return DispatchingJinjaLoader(self)
 
-    def select_jinja_autoescape(self, filename: str) -> bool:
+    def select_jinja_autoescape(self, filename: str | None) -> bool:
         """Returns ``True`` if autoescaping should be active for the given
         template name. If no template name is given, returns `True`.