when `set -o nounset` in Bash,
the warnings of unbound variables will break the bash completion.
use `kubectl` as example:
```sh
$ set -o nounset
$ my-cli <Tab>-bash: BASH_COMP_DEBUG_FILE: unbound variable
$
```
the warning break bash completion without any completion result,
and cause my cursor move to the newline.
Use `${variable:-}` substitution in Bash,
that assign an empty string as default for unbound variables to fix the warnings.
