Fix #809 Use showHelpOnEmpty

[mpilgrem]

For example, before the change, stack ls yields only:

Missing: COMMAND

Usage: stack.exe ls COMMAND [--setup-info-yaml URL]
                    [--snapshot-location-base URL] [--help]
  List command. (Supports snapshots, dependencies and stack's styles)

After the change, stack ls yields the more informative:

Usage: stack.exe ls COMMAND [--setup-info-yaml URL]
                    [--snapshot-location-base URL] [--help]
  List command. (Supports snapshots, dependencies and stack's styles)

Available options:
  --setup-info-yaml URL    Alternate URL or relative / absolute path for stack
                           dependencies
  --snapshot-location-base URL
                           The base location of LTS/Nightly snapshots
  --help                   Show this help text

Available commands:
  snapshots                View local snapshot (default option)
  dependencies             View the dependencies
  stack-colors             View stack's output styles
  stack-colours            View stack's output styles (alias for 'stack-colors')

Run 'stack --help' for global options that apply to all subcommands.

Also deletes the case:

Failure _ | null args -> withArgs ["--help"] (execParser parser)

as stack (without more) is now handled without it.

Also updates ChangeLog.md.

Tested by building and using Stack, including using Stack with scripts and stack script.

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.