[prompt] steeef: Fix git-info stashed zstyle
[prompt] steeef: Fix git-info stashed zstyle

Only define it if it's non-empty. New git-info changes will consider
zstyles to be defined even if they are empty.

joshuarubin/dotfiles,joshuarubin/dotfiles,Eriner/zim,joshuarubin/dotfiles,joshuarubin/dotfiles