copr: handling case when no argument is passed on cli

addressing:
dnf copr
Traceback (most recent call last):
  File "/usr/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 242, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 124, in _main
    cli.run()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1504, in run
    return self.command.run(self.base.extcmds)
  File "/usr/lib/python2.7/site-packages/dnf-plugins/copr.py", line 74, in run
    subcommand = extcmds[0]
IndexError: list index out of range
copr: list: print description

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>