fix: avoid trailing ? when queryString option resolves to empty string

[fooddilsn]

Summary

When the queryString option resolves to an empty string, getQueryString unconditionally prepends ?, resulting in a trailing ? on the upstream request path (e.g. /path? instead of /path).

This affects both forms:

• Function form: '?' + opts.queryString(...) when the function returns ''
• Object form: '?' + querystring.stringify(opts.queryString) when the object is empty {}

This is inconsistent with the default code path (no queryString option), which correctly returns '' when there's no query string.

Fix

Guard both paths to only prepend ? when the result is non-empty:

  const qs = opts.queryString(search, reqUrl, request)
  return qs ? '?' + qs : ''

Closes #458

[fox1t]

LGTM

[gurgunday]

Lgtm