fix(cli): guard nil file panic in generated download commands - #1241

Summary

A user reported that the latest CLI release panics on phrase locales download when a --tags filter matches zero translations:

$ phrase locales download --id en-US --project_id ... -t ... --file_format properties --tags foo
panic: runtime error: invalid memory address or nil pointer dereference
...
github.com/phrase/phrase-cli/cmd.initLocaleDownload.func1(...)
        /go/src/github.com/phrase/phrase-cli/cmd/api_locales.go:410 +0xdef

This is a sibling issue to #1235 (fixed in pull.go), but in a different code path: the standalone, per-endpoint phrase locales download command generated from openapi-generator/templates/cli/api.handlebars.

Root cause: the API returns 200 OK with an empty body when a filter matches no translations. The SDK's decode() leaves the *os.File result nil in that case with no error — a valid empty export, not a failed download. The generated command unconditionally called data.Close() / data.Name() on that nil file, causing a nil pointer dereference.

Since cmd/api_*.go files are generated (and gitignored — never committed), the fix has to live in the Handlebars template itself, which is shared by every generated command whose return type is *os.File. So this also covers other download-style endpoints beyond locale download, not just the one in the report.

Fix

Wrap the file-handling block in api.handlebars with a data != nil guard, mirroring the same fix already applied to copyToDestination in pull.go.

Test plan

• Added spec/locales_download_spec.rb, an integration spec that mocks the download endpoint returning 200 with an empty body and runs the actual built phrase-cli binary.
• Regenerated the CLI locally (npm run generate.go && npm run generate.cli) and confirmed the new spec fails with the exact reported panic against the pre-fix template, and passes after applying the fix.
• Confirmed normal (non-empty) downloads are unaffected.
• Full Ruby spec suite (38 examples) green.

🤖 Generated with Claude Code