when called without parameters, do as if --help was on the CLI

[UnixJunkie]

Currently, we have an obscure:

Traceback (most recent call last):
  File "/home/berenger/.local/bin/standardiser", line 11, in <module>
    sys.exit(main())
  File "/home/berenger/.local/lib/python3.6/site-packages/standardiser/bin/standardise_mols.py", line 75, in main
    input_type = os.path.splitext(config.infile)[1] # sdf or smi
  File "/usr/lib/python3.6/posixpath.py", line 122, in splitext
    p = os.fspath(p)
TypeError: expected str, bytes or os.PathLike object, not NoneType