[hotfix/open_option_no_default] Adding placeholder to command option

If the default value of the option at a command is null, you can set the placeholder that will be the value displayed by default.

before:
  --open="", -o  Open a url of default system in the preferred application

after:
  --open="application", -o  Open a url of default system in the preferred application
Fixing option --open in command `start/restart` not null