text: Title: jinja_loader type bug
  Body: I have
  ```Python
  from jinja2 import ChoiceLoader, FileSystemLoader
  
  
  app.jinja_loader = ChoiceLoader([
      FileSystemLoader(project_dir / 'templates/'),
      app.jinja_loader,
  ])
  ```
  
  There is a mypy error
  ```
  error: Incompatible types in assignment (expression has type "ChoiceLoader", variable has type "FileSystemLoader | None")  [assignment]
  error: List item 1 has incompatible type "FileSystemLoader | None"; expected "BaseLoader"  [list-item]
  ```
  
  There shouldn't be an error.
  
  Environment:
  
  - Python version: 3.12.1
  - Flask version: 3.0.1
  