Result variables: Minor improvements

[philippfromme]

Proposed Changes

Looking into #30 I found it hard to understand the logic. This PR aims to

• improve the documentation
• simplify the check for existing outputs
• improve code style

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

Good clean-up, thanks!