Fix nextvar.lua "concatenate nil" failure

[davydog187]

Tracked by plan: .agents/plans/A7-nextvar-suite.md

Status: blocked
Direction: A — Suite Triage

The plan file is the source of truth for goal, scope, success criteria, and implementation notes. Comments here are for discussion; the plan is for execution.

When this plan is picked up via /next-plan:

1. The branch named in the plan's frontmatter is created.
2. Plan status moves ready → in-progress → review.
3. A PR is opened with Closes #<this-issue> in the body.
4. Merge closes this issue.

For strategic context, see ROADMAP.md.