Cherry-pick "LibWeb: Avoid null dereference when performing mixed content checks"

[nico]

Previously, navigating to or from about:newtab caused a crash due to inadvertent null dereferences when checking whether a request or response to a request should be blocked as mixed content.

(cherry picked from commit 572ebe00eacd5aaeecc17207c75c6bf2327a3897)

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Cherry-picks https://github.com/LadybirdBrowser/ladybird/pull/161