Removed the None return in main gui function

[floffy-f]

Replace with [] return for "no document selected", to avoid crash on papis side when the user quits Rofi, with either the dedicated quit key (Alt+q) or the escape key.

The error used to be :

Traceback (most recent call last):
  File /home/<myusername>/.local/bin/papis, line 8, in <module>
    sys.exit(run())
  File /usr/lib/python3/dist-packages/click/core.py, line 1128, in __call__
    return self.main(*args, **kwargs)
  File /usr/lib/python3/dist-packages/click/core.py, line 1053, in main
    rv = self.invoke(ctx)
  File /usr/lib/python3/dist-packages/click/core.py, line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File /usr/lib/python3/dist-packages/click/core.py, line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File /usr/lib/python3/dist-packages/click/core.py, line 754, in invoke
    return __callback(*args, **kwargs)
  File /home/<myusername>/.local/lib/python3.10/site-packages/papis/commands/edit.py, line 101, in cli
    documents = papis.cli.handle_doc_folder_query_all_sort(query,
  File /home/<myusername>/.local/lib/python3.10/site-packages/papis/cli.py, line 162, in handle_doc_folder_query_all_sort
    documents = [doc for doc in papis.pick.pick_doc(documents) if doc]
TypeError: 'NoneType' object is not iterable