Running ntia-checker without arguments fails with non-useful error

[lumjjb]

Running ntia-checker without arguments provides an error message that isn't helpful. (i for a while thought i had library issues), should have a more useful error. Will create a PR.

➜  ntia-conformance-checker git:(main) python3 ntia_conformance_checker/main.py
Traceback (most recent call last):
  File "/Users/lumb/git/ntia-conformance-checker/ntia_conformance_checker/main.py", line 71, in <module>
    main()
  File "/Users/lumb/git/ntia-conformance-checker/ntia_conformance_checker/main.py", line 51, in main
    sbom = SbomChecker(args.file)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/ntia_conformance_checker/sbom_checker.py", line 20, in __init__
    self.doc = self.parse_file()
               ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/ntia_conformance_checker/sbom_checker.py", line 37, in parse_file
    if not os.path.exists(self.file):
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen genericpath>", line 19, in exists
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType