fix(ZSH Completions): fixes an issue where zsh completions caused panics if there were no subcommands
fix(ZSH Completions): fixes an issue where zsh completions caused panics if there were no subcommands

Closes #754

kbknapp/clap-rs,clap-rs/clap,clap-rs/clap,kbknapp/clap-rs,clap-rs/clap