[BUG] test-runner viewport can't be null

```ts
    {
      name: 'Chromium',
      use: {
        browserName: 'chromium',
        // Make Chromium maximized
        // @ts-ignore
        viewport: null,
        launchOptions: {
          headless: false,
          args: ["--start-maximized"]
        },
      },
    },
  ],
```

See the ts-ignore.

If we go with the #7221 approach, then we don't have to fix that.

