Unify WebBrowser API in regards to nullable bodies

[JustArchi]

Commit 6178b12 introduced two new request options which require response content to be nullable to account for it not being parsed.

While the above change was non-breaking, it further complicates ASF logic which now has two functions that do exactly the same apart from one small detail and different response type.

It makes sense to unify those two, and while at it bullet-proof for similar problems with HtmlDocumentResponse where we also might need to account for null/empty HTML on error.

This actually is not a breaking change per-se. Warnings will be emitted for plugin creators, but old code should still work fine without recompilation, as it was just annotation change. In terms of usage, null body might be returned only when explicitly asking for it with new request options, so even previous use cases remain valid.

I'm postponing it until next release cycle.

[Abrynos]

I trust you found all the code where we check response.Content and needed to add either the ? or the ! operator, because I'm not searching for those 😅