[23.0] Fix folder listing via file browser

[mvdbeek]

Added in #15340 by accident.
Fixes:

RuntimeError: No active exception to reraise
  File "galaxy/web/framework/decorators.py", line 337, in decorator
    rval = func(self, trans, *args, **kwargs)
  File "galaxy/webapps/galaxy/api/library_contents.py", line 130, in index
    for content in traverse(library.root_folder):
  File "galaxy/webapps/galaxy/api/library_contents.py", line 84, in traverse
    can_access, folder_ids = trans.app.security_agent.check_folder_contents(
  File "galaxy/model/security.py", line 1510, in check_folder_contents
    raise

in https://sentry.galaxyproject.org/share/issue/38df4490ff61415090c319ad0e16f1fa/

You can trigger this in the UI using the "Browse Files" if you're not an admin.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.