v3: Cant extend config

Environment

Nitro 3.0.260311-beta

Describe the bug

Extending a config object throws:

Cannot extend config from { ... }

Reproduction

Reproduction: https://github.com/MickL/nitro-extend-config-bug

nitro.config.ts:

import { defineConfig } from 'nitro';
import { baseConfig } from './base.config.ts';

export default defineConfig({
  extends: baseConfig,
  serverDir: './',
});

base.config.ts:

import { defineConfig } from 'nitro';

export const baseConfig = defineConfig({
  runtimeConfig: {
    databaseUrl: '',
  },
});