[makeotf] traceback with no input file

[frankrolf]

When makeotf is run without any input file, the following traceback happens:

Traceback (most recent call last):
  File "/usr/local/bin/makeotf", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.7/site-packages/afdko/makeotf.py", line 2712, in main
    setMissingParams(makeOTFParams)
  File "/usr/local/lib/python3.7/site-packages/afdko/makeotf.py", line 1790, in setMissingParams
    if not os.path.exists(inputFilePath):
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/genericpath.py", line 19, in exists
    os.stat(path)
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType