Only log experiments warning if user actually opted in to an experime…

…nt(s) (#34413)

Currently if you have a Next config like the following:

```js
module.exports = {
  experimental: {},
},
```

You are presented with the warning for experimental features, even though you haven't actually enabled any experiments. This PR checks there's at least one key in the `experimental` object before logging the warning.

packages/next/server/config.ts

@@ -67,8 +67,9 @@ function assignDefaults(userConfig: { [key: string]: any }) {
 
       if (
         key === 'experimental' &&
-        value !== undefined &&
-        value !== defaultConfig[key]
+        value !== defaultConfig[key] &&
+        typeof value === 'object' &&
+        Object.keys(value).length > 0
       ) {
         experimentalWarning()
       }