Whenever there's a risk that we will reference a shell variable in a
script run with `set -u` (including libraries that we sources from
such scripts) we must reference the variable in a way so it gets a
default value, i.e. `${VAR:-}` to give it the empty string as the
default (which is what generally makes sense).

In particular, it's quite clear that in all cases where we do
something like `[ -n "${VAR}" ]` or `[ -z "${VAR}" ]`, where an
expected value is the empty string, which could mean that the variable
is not initialized, we should give such a default (as the empty
string).