LibWeb: Skip HTMLLinkElement resource fetching for documents without BC
Fixes crashing after following steps:
1. Open https://github.com/SerenityOS/serenity
2. Click on "Pull requests" tab
The problem was `navigable` null pointer dereferencing in
`decode_favicon()`. But navigable is null because the document was
created by `parseFromString()` DOMParser API.
With this change we skip fetching initiated by HTMLLinkElement if
document does not have a browsing context:
- Favicon is not displayed for such documents so no need to fetch.
- Stylesheets fetching won't affect such document because style or
layout does not run for them.
Aliaksandr Kalenik