Support set -u in parent script

[AlexanderYastrebov]

Do not fail when parent script has set -u option.

See

• https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html
• https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
• http://mywiki.wooledge.org/BashFAQ/112

• [x] a GitHub Issue was opened for this feature / bug.
• [ ] test coverage was added (Cucumber or Spock as appropriate).

Fixes #597

[fredjaya]

Would be good to have this supported!

[AlexanderYastrebov]

@marc0der PTAL