Added check for main folder being found to prevent error

[HugovanSchalm]

Addresses #40.

I also encountered this error when syncing:

Traceback (most recent call last):
  File "/home/hugo/.local/bin/canvas", line 8, in <module>
    sys.exit(entry())
             ~~~~~^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/bin/canvas.py", line 186, in entry
    run_canvas_sync()
    ~~~~~~~~~~~~~~~^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/bin/canvas.py", line 132, in run_canvas_sync
    main_menu(settings)
    ~~~~~~~~~^^^^^^^^^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/bin/canvas.py", line 154, in main_menu
    do_sync(settings, "")
    ~~~~~~~^^^^^^^^^^^^^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/bin/canvas.py", line 170, in do_sync
    synchronizer.sync()
    ~~~~~~~~~~~~~~~~~^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/CanvasSync/entities/synchronizer.py", line 128, in sync
    course.sync()
    ~~~~~~~~~~~^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/CanvasSync/entities/course.py", line 160, in sync
    self.add_files_folder()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/hugo/.local/share/pipx/venvs/canvassync/lib/python3.13/site-packages/CanvasSync/entities/course.py", line 116, in add_files_folder
    main_folder[u"name"] = u"Other Files"
    ~~~~~~~~~~~^^^^^^^^^
TypeError: 'NoneType' object does not support item assignment

I think one of my courses has a weird file structure and thus has no course files folder. This check fixes it and allows that course to successfully sync.