[ruby] fix: HEAD requests (#9436)

When WEBrick receives `HEAD` requests it discards the body (i.e.
`req.body.nil? => true`), this causes Vercel to throw a
`BODY_NOT_A_STRING_FROM_FUNCTION` since it is expecting the serverless
function to respond with a string in the body.

---------

Co-authored-by: Nathan Rajlich <n@n8.io>
Co-authored-by: Steven <steven@ceriously.com>

packages/ruby/vc_init.rb

@@ -73,7 +73,7 @@ def webrick_handler(httpMethod, path, body, headers)
   {
     :statusCode => res.code.to_i,
     :headers => res_headers,
-    :body => res.body,
+    :body => res.body.nil? ? "" : res.body,
   }
 end