optimized-images calculation incorrect for cached images

**_EDIT (paul dec 2021)_**: the first part of this thread is working-as-intended but the real bug emerges here: https://github.com/GoogleChrome/lighthouse/issues/12465#issuecomment-852896577

--------

#### Provide the steps to reproduce

Running

```ts
const puppeteer = require('puppeteer');
const lighthouse = require('lighthouse');

async function run() {
const browser = await puppeteer.launch({
headless: false,
defaultViewport: null,
});

await lighthouse('https://www.example.com/', {
port: (new URL(browser.wsEndpoint())).port,
output: 'json',
logLevel: 'info',
}, {
passes: [{
passName: 'defaultPass',
// useThrottling: true,
recordTrace: true,
gatherers: [
'viewport-dimensions',
'dobetterweb/optimized-images',
'image-elements'
]
}],
audits: [
'byte-efficiency/uses-responsive-images'
]
});

await browser.close()
}

run()
```

#### What is the current behavior?

Observing the logs, I only see
```
status Cleaning browser cache +4ms
```
if I uncomment `// useThrottling: true,`

#### What is the expected behavior?

Lighthouse cleans browser cache, even if throttling is not enabled on a pass. The code that causes this is here: https://github.com/GoogleChrome/lighthouse/blob/145e24c7026c7ce000ebd4552b85774f8520c42e/lighthouse-core/gather/gather-runner.js#L758

#### Environment Information
* Lighthouse version: 7.4.0