ShellCheck warned me of a very similar issue in a script of mine recently. In that script I ran the following:

  rm -rf "${FOO}/"

ShellCheck helpfully warned me that that could have disastrous consequences when FOO is unset, instead it advised me to use this:

  rm -rf "${FOO:?}/"

I'd also like to point out that ShellCheck is in the AUR for users of Arch Linux[0], and that there's a handy plugin which I use for the Atom editor[1].

[0] https://aur.archlinux.org/packages/shellcheck-git/

[1] https://atom.io/packages/linter-shellcheck