Fix bug which led to unboud variable forced by set -u (#115)
Fix bug which led to unboud variable forced by set -u (#115)

When using set -u (and the init script is on set -eu) one can not
test a variable using [[ -z "$var" ]] because the var might be
_not_ set which will cause an exit 1.

This happens if someone tries to use the shield-mysql-addon with
LD_LIBRARY_PATH, the monit script exits with:

    /var/vcap/jobs/shield-agent/bin/shield-agent:
    line 53: LD_LIBRARY_PATH: unbound variable

Explanation: https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash
(i had to google... )
starkandwayne/shield-boshrelease,starkandwayne/shield-boshrelease