[docs] Avoid next config validation warning

[cherniavskii]

Documentation / Examples

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• The examples guidelines are followed from our contributing doc

Next.js now validates next.config.js and setting assetPath to an empty string results in the following warning:

  {
    "instancePath": "/assetPrefix",
    "schemaPath": "#/properties/assetPrefix/minLength",
    "keyword": "minLength",
    "params": {
      "limit": 1
    },
    "message": "must NOT have fewer than 1 characters"
  }

This was raised in #39161 (comment)