fix(input-variable-helper): correctly handle missing input

[barmac]

Which issue does this PR address?

This fixes an error if you remove all inputs from a decision.

Definition of Done

• [ ] corresponds to the design principles
• [ ] corresponds to the code standards
• [ ] passes Continuous Integration checks
• [ ] available as feature branch on GitHub
  • [ ] contains the cleaned up commit history
  • [ ] commit messages satisfy our commit message guidelines
  • [ ] a single commit closes the issue via Closes #issuenr