Support for bash strict mode

When running `dependency-check.sh` from a bash script which uses the highly popular "strict mode" though [set](https://www.gnu.org/software/bash/manual/bash.html#index-set), eg `set -euo pipefail`, an error is thrown because `dependency-check.sh` checks for a value in the (usually unset) variable `JAVACMD` with the message `dependency-check/bin/dependency-check.sh: line 75: JAVACMD: unbound variable`.
This could be remedied by substitution, eg replacing `[ -z "$JAVACMD" ]` with `[ -z "${JAVACMD:-}" ]` (similarly for other potentially unbound variables), which circumvents the unbound variable errors that bash throws when `set -u` is used