[sean0407]

Summary

${VAR+word}   -> use word when VAR is set
${VAR:+word}  -> use word only when VAR is set and nonempty

• Closes ${var:+FOO} mishandles empty vars #53.
• Keep + set-only behavior and make :+ check the variable's value before expanding its alternate word.
• Add regression coverage for an empty variable with a nonempty alternate, including that the alternate is not evaluated when the variable is empty.

Evidence

• Before: the isolated regression failed: ${EMPTY:+hello} returned hello instead of an empty string.
• After: the regression and parser suite pass.
• Changed files: parse/node.go, parse/parse_test.go.
• Checks run: gofmt check passed; go test passed; go test ./parse passed; go test -vet=off ./... passed; git diff --check passed. The project patch policy accepted 2 files and no binary changes.

Merge Danger

Door: one-way

Blast Radius: Low

The change affects only ${VAR:+word} when VAR is set to an empty value. ${VAR+word} remains set-only.