Remove unnecessary check: item cannot be None

[jdavcs]

The value of parent_folder cannot be None:

• _create_folder is called from galaxy.api.library_contents which raises an exception if the value is missing.
• if the value were indeed None, the subsequent call to parent_folder.add_folder(new_folder) would raise an error.

Bug discovered via mypy after adding type annotations in #16434.

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.