Don't write_description for subcommands without configurable options

[paddy-hack]

I added a subcommand to my application and added an non-configurable option to it.

When calling app.config_to_str(true, true); this led to the description for the subcommand to be included in the output even though there are no configurable options for the command. This is confusing.

[paddy-hack]

Actually, you don't even have to add any options to the subcommand to trigger this.

Note, the subcommand itself is, per default, not "configurable" (triggerable from the configuration file, really).