[prompt] change unset variable test
[prompt] change unset variable test

The old test, [[ ${var} ]] tests for unset AND zero-length strings.

Checking for zero length strings probably isn't that important. It is
enough to ensure that the variable is set.

Closes #18

joshuarubin/dotfiles,Eriner/zim,gyakovlev/zim,joshuarubin/dotfiles,hlidotbe/zim,joshuarubin/dotfiles,joshuarubin/dotfiles