Adding a Default Action in 12-cmdline.md solution fails on piped stdin

[rshpeley]

Solution readings09.py fails on piped stdin and no action argument:

python ../code/readings_09.py < small-01.csv
Traceback (most recent call last):
  File "../code/readings_09.py", line 33, in <module>
    main()
  File "../code/readings_09.py", line 7, in main
    action = sys.argv[1]
IndexError: list index out of range

This error occurs because there is no argument for action or filename. The fix adds a conditional test to handle the case where no args exist.

The readings_09.py file in the code folder would also need to be updated for this fix.

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