Relax restriction of images.path in next.config.js (vercel#30741)

The built-in `imgixLoader()` unexpectedly works with `image.path = ''` and `src` as an absolute URL.

So we need to relax this restriction for now until we can officially deprecate built-in loaders (eventually they can all be implemented in userland via loader prop).

- Follow up to vercel#30080

packages/next/server/config.ts

@@ -292,7 +292,7 @@ function assignDefaults(userConfig: { [key: string]: any }) {
     if (
       images.loader !== 'default' &&
       images.loader !== 'custom' &&
-      !(images.path || '').startsWith('http')
+      images.path === imageConfigDefault.path
     ) {
       throw new Error(
         `Specified images.loader property (${images.loader}) also requires images.path property to be assigned to a URL prefix.\nSee more info here: https://nextjs.org/docs/api-reference/next/image#loader-configuration`