You don't have to quote variables inside `[[ ]]`.

```
$ FOO="BAR"
$ [[ -z $FOO ]] && echo yep || echo nope
nope
$ FOO=""
$ [[ -z $FOO ]] && echo yep || echo nope
yep
$ unset FOO
$ [[ -z $FOO ]] && echo yep || echo nope
yep
```