> This is because it doesn't distinguish between a variable that is unset and a variable that is set to the empty string.

Yeah, this is insane and the default settings of bash are quite horrible. You can set some bash options in the beginning to avoid such problems. For the StackOverflow problem "set -o nounset" can be a good safeguard. Other options to look into to safeguard bash scripts from some insane behavior are "errexit" and "pipefail". Even though setting these is good practice, it doesn't stop some stuff that could be expected to be stopped by these. These options are not inherited to subshells invoked by command substitution and the likes. They wouldn't have stopped this nasty steam bug [1] for example. There are other nasty quirks too, it's really hard to unfck bash.

[1] https://github.com/ValveSoftware/steam-for-linux/issues/3671...