OXC.bundle/2 with sourcemap: true hard-fails when Rolldown omits map for empty output

[mtarnovan]

Summary

OXC.bundle/2 currently returns an error when sourcemap: true and Rolldown does not attach a map (e.g. empty or effectively-empty bundle output):

{:error, [%{message: "Rolldown did not produce a source map"}]}

This bubbles up to Volt builds and can fail otherwise-valid builds.

Environment

• oxc 0.10.0
• Elixir 1.19.4 / OTP 28
• macOS Darwin 25.4.0

Minimal Repro

mix run -e 'files=[{"main.js", ""}]; IO.inspect(OXC.bundle(files, entry: "main.js", sourcemap: true, format: :iife))'

Actual

{:error, [%{message: "Rolldown did not produce a source map"}]}

Expected

One of:

1. Return success with sourcemap: nil / omitted map for this edge case, or
2. Handle this case in a non-fatal way so callers can still receive generated code.

Additional context

Sourcemap disabled works as expected:

mix run -e 'files=[{"main.js", ""}]; IO.inspect(OXC.bundle(files, entry: "main.js", sourcemap: false, format: :iife))'
# => {:ok, "(function() {\n\n\n})();"}

Potential throw site appears in native/oxc_ex_nif/src/bundle.rs:

let sourcemap_json = chunk
    .map
    .as_ref()
    .map(oxc_sourcemap::SourceMap::to_json_string)
    .ok_or_else(|| vec!["Rolldown did not produce a source map".to_string()])?;

Happy to test any proposed patch quickly.