[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>

test/lib/deployment/test-deployment.js

@@ -384,7 +384,7 @@ async function fetchDeploymentUrl(url, opts) {
   for (let i = 0; i < 50; i += 1) {
     const resp = await fetch(url, opts);
     const text = await resp.text();
-    if (text && !text.includes('Join Free')) {
+    if (typeof text !== 'undefined' && !text.includes('Join Free')) {
       return { resp, text };
     }