Normalize optional catch-all fallback: true page params (vercel#17551)

This makes sure to normalize the params for optional catch-all routes on Vercel since for `fallback: true` pages the `[[...paramName]]` value will be provided for the undefined/root param which needs to be normalized. 

Tests have been added in vercel/vercel#5247 and were manually tested with the changes in this PR with https://github.com/ijjk/next-update-loader

Fixes: vercel#17220
x-ref: vercel/vercel#5247

Author: ijjk

packages/next/build/webpack/loaders/next-serverless-loader.ts

@@ -89,7 +89,12 @@ const nextServerlessLoader: loader.Loader = function () {
               (!value || (
                 Array.isArray(value) &&
                 value.length === 1 &&
-                value[0] === 'index'
+                ${
+                  ''
+                  // fallback optional catch-all SSG pages have
+                  // [[...paramName]] for the root path on Vercel
+                }
+                (value[0] === 'index' || value[0] === \`[[...\${key}]]\`)
               ))
             ) {
               value = undefined