parse_log.py bugfix: don't attempt to write CSV if there are no lines to write

[dgolden1]

An empty dict_list, which contains the contents of the to-be-written CSV, can occur if, e.g., testing is never performed in the log.

Previously, this yielded an error like:

Traceback (most recent call last):
  File "./parse_log.py", line 201, in <module>
    main()
  File "./parse_log.py", line 197, in main
    test_dict_list, delimiter=args.delimiter)
  File "./parse_log.py", line 146, in save_csv_files
    write_csv(test_filename, test_dict_list, delimiter, verbose)
  File "./parse_log.py", line 161, in write_csv
    dict_writer = csv.DictWriter(f, fieldnames=dict_list[0].keys(),
IndexError: list index out of range

[dgolden1]

@shelhamer any interest in merging this minor log parser patch?

[shelhamer]

Sure, thanks for the patch.