Report: add more robust handling of default property values stripped by LHR proto

### FAQ

- [X] Yes, my issue is not about [variability](https://github.com/GoogleChrome/lighthouse/blob/master/docs/variability.md) or [throttling](https://github.com/GoogleChrome/lighthouse/blob/master/docs/throttling.md).
- [X] Yes, my issue is not about a specific accessibility audit (file with [axe-core](https://github.com/dequelabs/axe-core) instead).

### URL

https://investmentu.com

### What happened?

I've set up an LHCI server with PSI auto collect.
But I'm getting one tiny issue related to `https://googlechrome.github.io/lighthouse/viewer/`.

When I click to open any report, the view complains:

```
Uncaught Error: Error rendering report: Cannot read property 'startsWith' of undefined
```

Debugging it, I "solved" this issue by setting the second parameter of this function to an empty string as default.


https://user-images.githubusercontent.com/1340508/128362112-8f36fc61-22a6-48b2-a9c9-f28c179d2bd8.mp4

https://user-images.githubusercontent.com/1340508/128362114-a33d036f-8a5f-4229-8ad3-83b11f74adc2.mp4

### What did you expect?

The report should be shown properly.

### What have you tried?

I've used local overwrites to debug and found the solution.

### How were you running Lighthouse?

PageSpeed Insights

### Lighthouse Version

8.0.0

### Chrome Version

_No response_

### Node Version

_No response_

### Relevant log output

_No response_