Fix logic around `brew install`
Fix logic around `brew install`

Previously, even if there were no missing formula, the `list_formula`
string would contain whitespace (used to separate the formula names).
This resulted in `brew install` being run without any formula provided.

To avoid this, only create the `list_formula` string when
`missing_formula` is not an empty array.

micahwood/linux-dotfiles,lgaechter/dotfiles,micahwood/linux-dotfiles,intiocean/dotfiles,sdillingham/dotfiles,SiTaggart/dotfiles,codfish/dotfiles,senorflor/dotfiles,seiryu9/dotfiles,micahwood/linux-dotfiles,marc0uk/dotfiles,pmacom/dotfiles,necolas/dotfiles,Stubby/dotfiles,bronxct1/dotfiles,Gastove/dotfiles,pertoyra/dotfiles,nick-thompson/dotfiles,jmyser/dotfiles,saclark/dotfiles,seiryu9/dotfiles,Stubby/dotfiles,codfish/dotfiles,SBoudrias/dotfiles,senorflor/linux-dotfiles,micahwood/linux-dotfiles,micahwood/linux-dotfiles,bronxct1/dotfiles