Support `set -u` on init.sh and before-task.sh

fix for "unbound variable" error when I use `set -u` on init.sh or before-task.sh

```sh
$ bashing new foo
Initializing ./foo ...
Successfully initialized './foo'.
$ cd foo
$ echo 'set -u' > src/init.sh
$ bashing deploy
Creating /home/kui/toybox/foo/target/foo-0.1.0-SNAPSHOT.sh ...
Uberbash created successfully.
Deploying to /home/kui/toybox/foo/bin/foo ...
Deployed successfully.
$ bin/foo
bin/foo: line 20: $1: unbound variable
```