Custom gatherer not executed

Hey there,

I'm trying to create a custom gatherer that is executed when Lighthouse is running.

I have the following setup:
```js
const chrome = await launch({
chromeFlags: ["--headless"],
});

const result = await lighthouse(
"https://www.example.com",
{
port: chrome.port,
logLevel: "warn",
output: "json",
},
{
extends: "lighthouse:default",
artifacts: [
{
id: "my-artifact",
gatherer: "./custom-gatherer.js",
},
]
}
```

I see that the gatherer file is loaded but not executed.
As soon as I remove the `extends: "lighthouse:default"`, it's running.

Is this intended?

Another thing I noticed:
Even if I load the full config (instead of just doing `extends: "lighthouse:default"`), it's not running. The only way I made it work was to create a fake audit with a fake category, only then, the gatherer was running.

Appreciate any points or insights, it would be great to avoid the fake audit/category!

Start with the provided Lighthouse configuration, `custom-gatherer.js`, and the behavior of `extends: "lighthouse:default"`; compare it with the full config and the fake audit/category workaround. Confirm that a custom gatherer is executed without requiring a fake audit or category, and add or update coverage for that configuration path.

Written by the indexing model from the issue text.